replace deprecated getQueryPart() in ContextMapper

• Doctrine 4 removed getQueryPart() (PR)
• … and marked it as deprecated in 3.8 (PR)
• So it is also marked as deprecated in NCs abstraction (PR) as of NC 30

Money quote:

This function is going to be removed with the next Doctrine/DBAL update and we can not fix this in our wrapper. Please track the details you need, outside the object.

We use it in one place to figure out whether a WHERE condition is already set to know whether we either have to use where() or andWhere() in that method. Cf. https://github.com/nextcloud/tables/blob/1849033dbc6061f053dca4012dc2c4fd01ec61be/lib/Db/ContextMapper.php#L316-L320

Server wants to bump to Doctrine 4 in the upcoming version.

• Figure out whether we can solve it with available methods
• If so, implement the changes in a backwards compatible manner
• If not so, bring up necessary changes to the IQueryBuilder abstraction in server and follow up

Side effect: currently there is potential deprecation log spam against >= NC 30.

Start with lib/Db/ContextMapper.php around lines 316-320 and review the Doctrine DBAL deprecation and removal references in the issue. Check whether available query-builder methods support the existing behavior while preserving compatibility. If not, follow the required changes to the server-side IQueryBuilder abstraction; done means the deprecated call is no longer needed and the compatibility path is addressed.