feat(platform/bitbucket): add support for fetching release notes (#22094)

lib/constants/platforms.ts

lib/modules/platform/bitbucket/schema.ts

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

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

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

...orkers/repository/update/pr/changelog/bitbucket/source.ts

lib/workers/repository/update/pr/changelog/release-notes.ts

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

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