refactor: targetBranch mandatory in platform.createPr (#6829)

lib/platform/azure/index.spec.ts

lib/platform/azure/index.ts

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

lib/platform/bitbucket-server/index.ts

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.spec.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/workers/repository/error.ts

lib/workers/repository/onboarding/pr/index.ts