Fix Update Changelog workflow push failure

[swilla]

Summary

• The Update Changelog workflow (triggered on release: released) failed because it pushed to github.event.release.target_commitish, which can be a raw commit SHA rather than a branch name. git-auto-commit-action then tried to push to a branch named after that SHA and git rejected it with "not a full refname."
• Changed both the checkout ref and the commit action's branch to use github.event.repository.default_branch instead, so the changelog update always targets a real branch.
• Added the missing v2.4.1 changelog entry that failed to commit during the broken run.

See the failing run: https://github.com/TappNetwork/Filament-Forum/actions/runs/28719111134/job/85165569236

Test plan

• Cut a new release and confirm the Update Changelog workflow succeeds and pushes the CHANGELOG.md update to main