[ci] Remove duplicate backport action (elastic#190643)

Depends on elastic/kibana-github-actions#40

This action is no longer needed, we've merged auto-backport into the
current workflow.

.github/workflows/on-merge.yml

@@ -10,7 +10,6 @@ jobs:
     runs-on: ubuntu-latest
     if: |
       github.event.pull_request.merged == true
-      && !contains(github.event.pull_request.labels.*.name, 'auto-backport')
       && (
         (
           github.event.action == 'labeled' && (
@@ -19,6 +18,7 @@ jobs:
             || github.event.label.name == 'backport:current-major'
             || github.event.label.name == 'backport:all-open'
             || github.event.label.name == 'backport:version'
+            || github.event.label.name == 'auto-backport'
           )
         )
         || (github.event.action == 'closed')

dev_docs/contributing/how_we_use_github.mdx

@@ -207,10 +207,11 @@ it's notable and affects our users) and which section it appears in. For example
 
 The following labels are related to backporting PRs:
 
-- `auto-backport`: Automatically backport this PR (to the branches related to
+- `backport:version`: Automatically backport this PR (to the branches related to
    version labels) after it's merged. Requires adding desired target versions labels.
 - `backport:prev-minor`: Automatically backport to one lower minor version.
-- `backport:prev-major`: Automatically backport to the latest minor version of one lower major version.
+- `backport:prev-major`: Automatically backport to all minor version of one lower major version.
+- `backport:current-major`: Automatically backport to all minor version of the current major version.
 - `backport:all-open`: Automatically backport to all generally available versions. This functionally is equivalent to backport:prev-major at the time of writing.
 - `backport:skip`: This PR does not require backporting.
 - `backport`: This PR was backported (added by CI).

dev_docs/tutorials/submit_a_pull_request.mdx

@@ -55,7 +55,7 @@ If this is a new branch, you will see a link in your terminal that points you di
    2. Features: Add labels for any relevant feature areas, e.g. `Feature:Development`
    3. Team: Most PRs should have at least one `Team:` label. Add labels for teams that should follow or are responsible for the pull request.
    4. Release Note: Add `release_note:skip` if this pull request should not automatically get added to release notes for Kibana
-   5. Auto Backport: Add `auto-backport` if you'd like your pull request automatically backported to all labeled versions.
+   5. Auto Backport: Add `backport:version` if you'd like your pull request automatically backported to all labeled versions.
 6. Submit the pull request. If it's not quite ready for review, it can also be submitted as a Draft pull request.
 
 ![Screenshot of Compare and pull request header](../assets/pr_header.png)