Raise default CI variable limits

What does this MR do and why?

Data gathering revealed that the maximum number of CI variables currently used by projects and groups on gitlab.com is much higher than we originally suspected. This commit raises the default variable limits so that enforcing them on the backend will not be a breaking change

Migration output

db:migrate:main

db:rollback:main

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #373289 (closed)