git-mirrors promoted from experiment, enabled by git-mirrors-path

[pda]

This draft pull request promotes the git-mirrors functionality from experiment to standard. It still requires git-mirrors-path to be configured, otherwise it remains disabled. This makes it pretty safe to promote from experiment status, with a few caveats below.

elastic-ci-stack-for-aws consideration

Note that elastic-ci-stack-for-aws prior to https://github.com/buildkite/elastic-ci-stack-for-aws/pull/698 does provide a git-mirrors-path even when the stack chooses not to enable the experiment. That means shipping this PR could result in surprising/disruptive behaviour if it's used in an elastic-stack prior to that PR merged.

test coverage

This PR regrettably removes test coverage for git-mirrors, because that coverage was being driven externally via TEST_EXPERIMENTS: https://github.com/buildkite/agent/blob/f119ac1494f1c10ae5811c84f58a0c47a932ee89/bootstrap/integration/main_test.go#L38

… as well as special handling in BootstrapTester: https://github.com/buildkite/agent/blob/f119ac1494f1c10ae5811c84f58a0c47a932ee89/bootstrap/integration/bootstrap_tester.go#L111-L119

… and a special CI test entrypoint: https://github.com/buildkite/agent/blob/f119ac1494f1c10ae5811c84f58a0c47a932ee89/.buildkite/steps/tests.sh#L9-L10

I think this PR should add some test coverage back in, but via a dedicated test which sets up a git-mirror-path at at least traverses the happy path of the code.