Upgrade shoulda-matchers gem to v6.4.0

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183355



Merged-by: Rajendra Kadam <rkadam@gitlab.com>
Approved-by: Marius Bobin <mbobin@gitlab.com>
Approved-by: Rajendra Kadam <rkadam@gitlab.com>
Reviewed-by: Rajendra Kadam <rkadam@gitlab.com>
Co-authored-by: Avielle Wolfe <awolfe@gitlab.com>