fix(github): skip forked repos when in fork mode (#27684)

docs/usage/self-hosted-configuration.md

lib/constants/error-messages.ts

lib/modules/platform/github/graphql.ts

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

lib/modules/platform/github/index.ts

lib/modules/platform/github/types.ts

lib/util/git/index.ts

lib/workers/repository/error.spec.ts

lib/workers/repository/error.ts

lib/workers/repository/result.ts