feat(limits): Add "branchConcurrentLimit" option (#8046)

docs/usage/configuration-options.md

lib/config/definitions.ts

lib/workers/branch/index.spec.ts

lib/workers/branch/index.ts

lib/workers/common.ts

lib/workers/global/limits.ts

lib/workers/pr/index.ts

lib/workers/repository/dependency-dashboard.ts

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

lib/workers/repository/process/limits.ts

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

lib/workers/repository/process/write.ts