Restrict access level for PrAT with custom roles

What does this MR do and why?

This merge request restricts the access level for project access tokens (PrATs) with custom roles. With the change, a user with a custom role can only create a PrAT with an access level that is less than or equal to their own access level. This change helps to prevent users from creating PrATs with higher access levels than they have themselves.

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MR acceptance checklist

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Related to https://gitlab.com/gitlab-org/gitlab/-/issues/431307