Add application limits to Ci::InstanceVariable

In !30156 (merged) we are adding global (instance level) variables for CI jobs.

We should add an upper limit of 25 on the number of variables that can be defined, because:

• these variables are loaded for each job
• Our UI creates/updates them in bulk.

Add the limit behind a feature flag so if it does break someone on-prem they can disable the flag to resolve breakage.