Unify branch existence validation in prepare-release action

[fr4nc1sc0-r4m0n]

Summary

• Merge the duplicate major and minor branch-existence checks in prepare-release into a single step
• Keep release-type-specific error messaging via RELEASE_TYPE
• Step title clarifies it runs for major or minor releases only

Test plan

• Verify prepare-release composite action YAML is valid
• Confirm major/minor release workflows still invoke the action unchanged

Notifies https://github.com/elastic/observability-robots/issues/3404

[github-actions]

🤖 GitHub comments

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• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[prodsecmachine]

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[mergify]

This pull request does not have a backport label. Could you fix it @fr4nc1sc0-r4m0n? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8.19 is the label to automatically backport to the 8.19 branch.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[fr4nc1sc0-r4m0n]

@v1v @ninalee12 Taking into account that the release process should be idempotent as defined in https://docs.google.com/document/d/1VxKftPLqC3EtkxtjWpkbHqfP0Oen9142Piri-yviTVw/edit?tab=t.0, should be manage these errors in a different way?

[ninalee12]

Hey @fr4nc1sc0-r4m0n, Sorry I saw this late!

I think for error messages, we can log them however if the branch was already created, we should exit 0 instead of exit 1.

From the recent minor bump 9.5.0 we saw that when re-running the centralized pipeline, a team may have already created their branch and completed their bump. When the pipeline is re-run, this step will fail if we exit 1 and block any downstream dependent teams. Since an existing branch in this context means "step already completed successfully," exiting 0 with a warning message is the safer, idempotent behavior.

[ninalee12]

I created a new PR on top of this branch to add in additional checks for idempotency as it's out of scope for this PR.

#21555

[v1v]

Hey Nina, I just left a few comments

[mergify]

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