Allow Dedicated customers to customize maximum instance-level CI/CD variable count

Proposal

Currently, GitLab Dedicated customers are limited to 25 instance-level CI/CD variables. Self-managed customers can change this limit via a Rails console command, but Dedicated customers do not have access to do so. I'm requesting that we add support for modifying this setting via the UI and/or API so that Dedicated customers who need more variables can make the necessary changes.

Implementation Table

Group	Issue Link
backend	Backend: Allow Dedicated customers to customize maximum instance-level CI/CD variable count
frontend	👈 You are here
documentation	Documentation: Allow Dedicated customers to customize maximum instance-level CI/CD variable count
Instrumentation	Issue Title