Limit CI workflow scope for pushes

[gbanasiak]

#811 worked as expected and now backported PRs are automatically tested.

However this showed another problem. There is a duplication of workflows, one is raised for a branch push, and another for PR activity. To eliminate this duplication, I am limiting on push triggers to master and [0-9]* branches. The latter is supposed to match <major>, <major>.<minor> and <major>.<minor>.<patch> notation supported by Rally.

Unfortunately GH does not support regexes but a subset of fnmatch syntax hence the overly broad pattern of [0-9]* which happens to work for us for now.

[esbenchmachine]

💚 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 and see the Github Action logs for details

[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!