refactor: remove platform.setBaseBranch (#7137)

lib/platform/azure/index.ts

...latform/bitbucket-server/__snapshots__/index.spec.ts.snap

lib/platform/bitbucket-server/index.spec.ts

lib/platform/bitbucket-server/index.ts

lib/platform/bitbucket/__snapshots__/index.spec.ts.snap

lib/platform/bitbucket/index.spec.ts

lib/platform/bitbucket/index.ts

lib/platform/common.ts

lib/platform/gitea/index.spec.ts

lib/platform/gitea/index.ts

lib/platform/github/index.ts

lib/platform/gitlab/__snapshots__/index.spec.ts.snap

lib/platform/gitlab/index.spec.ts

lib/platform/gitlab/index.ts

lib/util/git/__snapshots__/index.spec.ts.snap

lib/util/git/index.spec.ts

lib/util/git/index.ts

lib/workers/branch/index.ts

lib/workers/repository/init/index.ts

lib/workers/repository/onboarding/branch/index.spec.ts