feat(changelogs): prioritize markdown format for multiple file candidates (#25846)

...rs/repository/update/pr/changelog/bitbucket/index.spec.ts

...workers/repository/update/pr/changelog/bitbucket/index.ts

lib/workers/repository/update/pr/changelog/common.spec.ts

lib/workers/repository/update/pr/changelog/common.ts

...orkers/repository/update/pr/changelog/gitea/index.spec.ts

lib/workers/repository/update/pr/changelog/gitea/index.ts

lib/workers/repository/update/pr/changelog/github/index.ts

lib/workers/repository/update/pr/changelog/gitlab/index.ts

...kers/repository/update/pr/changelog/release-notes.spec.ts