Allow Dedicated customers to customize maximum instance-level CI/CD variable count
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### Proposal
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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
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| Group | Issue Link |
| ------ | ------ |
| ~backend | [Backend: Allow Dedicated customers to customize maximum instance-level CI/CD variable count](https://gitlab.com/gitlab-org/gitlab/-/issues/458023) |
| ~frontend | :point_left: You are here |
| ~documentation | [Documentation: Allow Dedicated customers to customize maximum instance-level CI/CD variable count](https://gitlab.com/gitlab-org/gitlab/-/issues/458027) |
| Instrumentation | [Issue Title](url) |
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