Limit the number of agent tokens created

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/120825



Merged-by: Kamil Trzciński <ayufan@ayufan.eu>
Approved-by: Kamil Trzciński <ayufan@ayufan.eu>
Approved-by: Phillip Wells <pwells@gitlab.com>
Reviewed-by: Kamil Trzciński <ayufan@ayufan.eu>
Reviewed-by: Phillip Wells <pwells@gitlab.com>
Co-authored-by: Pam Artiaga <partiaga@gitlab.com>