release-23.2: catalog/lease: optimize lease count query

[fqazi]

Previously, the query to count leases would end up scanning the entire system.leases table because of the addition of crdb_region as the first column on 23.1. This meant that scans not specifying a region would always be forced to scan the entire table. To address this, this patch will update the count query to specify the default region when working on single region system databases.

Fixes: #119456

Release note (bug fix): Schema changes with a large number of descriptors could end up doing full table scans on system.leases.

Release justification: a low risk fix that optimizes the leasing query for non-multi-region clusters, which was causing a regression for customers.

[blathers-crl[bot]]

Thanks for opening a backport.

Please check the backport criteria before merging:

• [ ] Backports should only be created for serious issues or test-only changes.
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• [ ] There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[rimadeodhar]

We should also post this in the backports channel.

[fqazi]

@mgartner @rimadeodhar TFTR!