feat: compare all branch authors when deciding if a branch is modified (#21558)

lib/modules/platform/default-scm.spec.ts

lib/modules/platform/default-scm.ts

lib/modules/platform/types.ts

lib/util/git/index.spec.ts

lib/util/git/index.ts

lib/workers/repository/config-migration/branch/rebase.ts

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

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

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