feat(platform): re-attempt platform automerge on github and gitlab (#26567)

docs/usage/configuration-options.md

lib/modules/platform/github/index.spec.ts

lib/modules/platform/github/index.ts

lib/modules/platform/gitlab/index.spec.ts

lib/modules/platform/gitlab/index.ts

lib/modules/platform/types.ts

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

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