Trim unnecessary ITs

[gbanasiak]

This removes unused stateful Buildkite pipeline, and changes serverless pipeline to be only triggered by Buildkite PR bot (via API). Serverless tests only make sense when run against master branch so this gets narrowed down too.

[esbenchmachine]

💔 All backports failed

Status	Branch	Result
❌	8.15	Backport failed because of merge conflicts

Manual backport

To create the backport manually run:

backport --pr 809

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details

[gbanasiak]

💚 All backports created successfully

Status	Branch	Result
✅	8.15

Note: Successful backport PRs will be merged automatically after passing CI.

Questions ?

Please refer to the Backport tool documentation

[esbenchmachine]

@gbanasiak
A backport is pending for this PR. Please add all required vX.Y version labels.

• If it is intended for the current Elasticsearch release version, apply the corresponding version label.
• If it also supports past released versions, add those labels too.
• If it only targets a future version, wait until that version label exists and then add it.
  (Each rally-tracks version label is created during the feature freeze of a new Elasticsearch branch).

Backporting entails:

1. Ensure the correct version labels exist in this PR.
2. Ensure backport PRs have backport label and are passing tests.
3. Merge backport PRs (you can approve yourself and enable auto-merge).
4. Remove backport pending label from this PR once all backport PRs are merged.

Thank you!