Change RELEASE_BRANCH_RE to MERGE_QUEUE_BRANCH_RE (#2726)

This regex is intended to detect the base branch for cherry-picks,
including merge queue commits. However, actual cherry-pick PRs don't use
this code path, as they already have the
`BUILDKITE_PULL_REQUEST_BASE_BRANCH` environment variable set. So
effectively this regex is only used to detect merge queue branches.

However, the current regex overmatches branches, including rolling
release branches (e.g. `release-10.0.0-pre.20260630.1rc1`). Historically
rolling release branches have used the default branch as the base. So
this PR changes the regex to narrowly match against merge queue
branches.