chore: enable GitHub merge queue

[anithapriyanatarajan]

Changes

Add merge_group trigger to CI workflows to support GitHub's merge queue feature. This ensures all PRs are tested in queue order against the latest main branch before merging, preventing race conditions.

The workflows will now run when PRs are added to the merge queue, in addition to regular pull_request events.

Similar to tektoncd/pipeline PR #9164

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

/kind cleanup

[vdemeester]

/lgtm

[tekton-robot]

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