fix: make baseBranch type non-optional in BranchConfig (#17920)

lib/workers/repository/cache.ts

lib/workers/repository/onboarding/pr/pr-list.spec.ts

lib/workers/repository/process/extract-update.spec.ts

lib/workers/repository/process/write.spec.ts

lib/workers/repository/process/write.ts

...itory/update/branch/execute-post-upgrade-commands.spec.ts

lib/workers/repository/update/branch/index.ts

lib/workers/repository/update/branch/reuse.spec.ts

lib/workers/repository/update/branch/reuse.ts

lib/workers/repository/update/pr/automerge.ts

lib/workers/repository/update/pr/body/changelogs.spec.ts

...kers/repository/update/pr/body/config-description.spec.ts

lib/workers/repository/update/pr/body/footer.spec.ts

lib/workers/repository/update/pr/body/header.spec.ts

lib/workers/repository/update/pr/body/index.spec.ts

lib/workers/repository/update/pr/body/notes.spec.ts

lib/workers/repository/update/pr/body/updates-table.spec.ts

lib/workers/repository/update/pr/index.spec.ts

lib/workers/types.ts

test/util.ts