Fix configuration of standby partitions

This change adds a new configuration option for controlling the maximum number of inactive partitions in Gitaly. Previously, this value was hardcoded to 2, but now it's configurable through the new transactions.max_inactive_partitions setting in the configuration file. The default value is set to 100 if not explicitly configured. The change includes appropriate tests to verify the sanitize logic works correctly.

Closes #6735 (closed)