graphics/qgis-ltr: Update to 3.28.7

Bugfix release. While here, also fix cmake configure:
If PostgreSQL server and client are present, the 'About' dialog
does not show the PostgreSQL version any more

Changelog: https://github.com/qgis/QGIS/compare/final-3_28_6%5E...final-3_28_7

Reported by: portscout, Repology
MFH: 2023Q2