Make stop method of the taskManager plugin async

[ersin-erdal]

towards: #189306

This PR fixes the Deleting current node has failed.errors mentioned in the above issue.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[mikecote]

Code LGTM, tested locally and worked as expected.

[pmuellr]

LGTM

Wondering if there's an easy way to test this in serverless before merging ...

[kibana-ci]

💛 Build succeeded, but was flaky

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[kibanamachine]

💔 All backports failed

Status	Branch	Result
❌	8.15	Backport failed because of merge conflicts

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Please refer to the Backport tool documentation

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[kibanamachine]

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[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add the label auto-backport or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 191218 locally

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 191218 locally

[kibanamachine]

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To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 191218 locally

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 191218 locally

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 191218 locally