ci(release): Default merge target to v2 (#3154)

Ensure release runs from the v2 line merge release branches back into v2
instead of falling back to the repository default branch.

This avoids post-publish merge conflicts when cutting previous-major
releases (e.g. 2.x) while master is on v3.

The workflow now sets a v2 default in dispatch inputs and also applies a
runtime fallback when invoking action-prepare-release.

Author: szokeasaurusrex

.github/workflows/release.yml

@@ -9,8 +9,9 @@ on:
         description: Force a release even when there are release-blockers (optional)
         required: false
       merge_target:
-        description: Target branch to merge into. Uses the default branch as a fallback (optional)
+        description: Target branch to merge into. Defaults to v2 for this branch
         required: false
+        default: v2
 jobs:
   release:
     runs-on: ubuntu-24.04