Bump rspec-mocks version to 3.12.6

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126152



Merged-by: Max Woolf <mwoolf@gitlab.com>
Approved-by: Max Woolf <mwoolf@gitlab.com>
Reviewed-by: Igor Drozdov <idrozdov@gitlab.com>
Co-authored-by: Igor Drozdov <idrozdov@gitlab.com>