chore(release): bump version to 3.9 as part of the feature freeze

[AndyZhang0707]

Summary

Bump the kong master version to 3.9 as part of the 3.8 feature freeze work.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

KAG-5216

Fix #[issue number]

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-13523-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-13523-to-master-to-upstream
git checkout -b cherry-pick-13523-to-master-to-upstream
ancref=$(git merge-base 9269195330e2a1ef7f6ae231d3063aa28e94403b 80d959dafa5a492e2f90cc072a1f7d55248d02ea)
git cherry-pick -x $ancref..80d959dafa5a492e2f90cc072a1f7d55248d02ea

[kikito]

Cherrypick/equivalent in https://github.com/Kong/kong-ee/pull/10013