graphics/qgis-ltr: Update to 3.10.12

Bugfix release [1][2].

[1] https://qgis.org/en/site/forusers/visualchangelog310/index.html
[2] https://github.com/qgis/QGIS/compare/final-3_10_11...final-3_10_12

If you miss features in QGIS-LTR that are actually
built into QGIS-LTR, you may need to enable additional
options in graphics/gdal, see pkg-message.

Approved by: arrowd, tcberner (mentors, implicit)