Squash housekeeping period options with a single option

Summary

With the introduction of OptimizeRepository RPC some of the housekeeping options become redundant.

Currently, we have 3 options that are responsible for how often a particular optimization is called:

• Incremental repack period: Number of Git pushes after which an incremental git repack is run.
• Full repack period: Number of Git pushes after which a full git repack is run.
• Git GC period: Number of Git pushes after which git gc is run.

With OptimizeRepository introduced, the frequency of a particular operation can't be controlled from Gitlab Rails anymore. Let's choose the lowest one out of these three (Incremental repack period) and use it as a value of how often OptimizeRepository is called.

Action steps

• Replace the three options on the UI with a single one
• Use housekeeping_incremental_repack_period column as a value of how often OptimizeRepository is called
• Update the documentation to reflect the current state of things

Availability and Testing

Please assign a SET for MR review, likely changes to the existing tests required. Possibly a new E2E if coverage deemed insufficient (to be evaluated).