[8.8] [RAM] Fix snooze scheduler timezone handling (#157338) (#158012)

# Backport

This will backport the following commits from `main` to `8.8`:
- [[RAM] Fix snooze scheduler timezone handling
(#157338)](#157338)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
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<!--BACKPORT [{"author":{"name":"Zacqary Adam
Xeper","email":"[email protected]"},"sourceCommit":{"committedDate":"2023-05-17T16:41:04Z","message":"[RAM]
Fix snooze scheduler timezone handling (#157338)\n\n##
Summary\r\n\r\nCloses #156535 \r\n\r\nThe Snooze Scheduler was failing
to properly save and load snoozes if\r\nthe user selected a timezone
other than the Kibana default. This is\r\nbecause the datepicker only
converts timestamp values between UTC and\r\nthe default Kibana
timezone.\r\n\r\nThis PR fixes the issue by offsetting all dates that
come in and out of\r\nthe scheduler UI relative to local time.\r\n\r\nTo
test, create a snooze like this on `main` and make sure you
select\r\ntimezone America/Los_Angeles. (If your local timezone is
equivalent to\r\nAmerica/Los_Angeles, select a different
timezone)\r\n\r\n<img width=\"426\" alt=\"Screenshot 2023-05-10 at 3 58
57
PM\"\r\nsrc=\"https://github.com/elastic/kibana/assets/1445834/ab95c47c-30a0-44d5-bd9d-45fdb929193d\">\r\n\r\nOn
`main`, editing the snooze will (erroneously) display the
wrong\r\ntimes:\r\n\r\n<img width=\"423\" alt=\"Screenshot 2023-05-10 at
4 03 24
PM\"\r\nsrc=\"https://github.com/elastic/kibana/assets/1445834/ff28ccdd-f491-43ce-87cd-d606c8d71c64\">\r\n\r\nRepeating
this process on this PR's branch will save a snooze with the\r\ncorrect
`dtstart` and consequently load the correct snooze
time.","sha":"213a69739f89bde0fefab284618968a77d1798b8","branchLabelMapping":{"^v8.9.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:fix","Team:ResponseOps","Feature:Alerting/RulesManagement","v8.9.0","v8.8.1"],"number":157338,"url":"https://github.com/elastic/kibana/pull/157338","mergeCommit":{"message":"[RAM]
Fix snooze scheduler timezone handling (#157338)\n\n##
Summary\r\n\r\nCloses #156535 \r\n\r\nThe Snooze Scheduler was failing
to properly save and load snoozes if\r\nthe user selected a timezone
other than the Kibana default. This is\r\nbecause the datepicker only
converts timestamp values between UTC and\r\nthe default Kibana
timezone.\r\n\r\nThis PR fixes the issue by offsetting all dates that
come in and out of\r\nthe scheduler UI relative to local time.\r\n\r\nTo
test, create a snooze like this on `main` and make sure you
select\r\ntimezone America/Los_Angeles. (If your local timezone is
equivalent to\r\nAmerica/Los_Angeles, select a different
timezone)\r\n\r\n<img width=\"426\" alt=\"Screenshot 2023-05-10 at 3 58
57
PM\"\r\nsrc=\"https://github.com/elastic/kibana/assets/1445834/ab95c47c-30a0-44d5-bd9d-45fdb929193d\">\r\n\r\nOn
`main`, editing the snooze will (erroneously) display the
wrong\r\ntimes:\r\n\r\n<img width=\"423\" alt=\"Screenshot 2023-05-10 at
4 03 24
PM\"\r\nsrc=\"https://github.com/elastic/kibana/assets/1445834/ff28ccdd-f491-43ce-87cd-d606c8d71c64\">\r\n\r\nRepeating
this process on this PR's branch will save a snooze with the\r\ncorrect
`dtstart` and consequently load the correct snooze
time.","sha":"213a69739f89bde0fefab284618968a77d1798b8"}},"sourceBranch":"main","suggestedTargetBranches":["8.8"],"targetPullRequestStates":[{"branch":"main","label":"v8.9.0","labelRegex":"^v8.9.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/157338","number":157338,"mergeCommit":{"message":"[RAM]
Fix snooze scheduler timezone handling (#157338)\n\n##
Summary\r\n\r\nCloses #156535 \r\n\r\nThe Snooze Scheduler was failing
to properly save and load snoozes if\r\nthe user selected a timezone
other than the Kibana default. This is\r\nbecause the datepicker only
converts timestamp values between UTC and\r\nthe default Kibana
timezone.\r\n\r\nThis PR fixes the issue by offsetting all dates that
come in and out of\r\nthe scheduler UI relative to local time.\r\n\r\nTo
test, create a snooze like this on `main` and make sure you
select\r\ntimezone America/Los_Angeles. (If your local timezone is
equivalent to\r\nAmerica/Los_Angeles, select a different
timezone)\r\n\r\n<img width=\"426\" alt=\"Screenshot 2023-05-10 at 3 58
57
PM\"\r\nsrc=\"https://github.com/elastic/kibana/assets/1445834/ab95c47c-30a0-44d5-bd9d-45fdb929193d\">\r\n\r\nOn
`main`, editing the snooze will (erroneously) display the
wrong\r\ntimes:\r\n\r\n<img width=\"423\" alt=\"Screenshot 2023-05-10 at
4 03 24
PM\"\r\nsrc=\"https://github.com/elastic/kibana/assets/1445834/ff28ccdd-f491-43ce-87cd-d606c8d71c64\">\r\n\r\nRepeating
this process on this PR's branch will save a snooze with the\r\ncorrect
`dtstart` and consequently load the correct snooze
time.","sha":"213a69739f89bde0fefab284618968a77d1798b8"}},{"branch":"8.8","label":"v8.8.1","labelRegex":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Co-authored-by: Zacqary Adam Xeper <[email protected]>