Fixed #22250 -- regression in gis compiler for non-concrete fields

Thanks to gwahl@fusionbox.com for the report.
Confirmed GEOS 3.4 support
Confirmed Spatialite 4 support
Added note in geo library docs
Dropped support for GEOS 3.1
Dropped support for GDAL 1.6
Removed unused imports.
Fixed #22329 -- Used label_tag() in some admin auth templates.

refs #17922.
Fixed #21858 -- Clarified 1.6 release note regarding M2M help text changes.

Thanks lee at semel.net for the report.
Unit testing Postgis version check

Refs #22334.
Skipped PostGIS version tests when psycopg2 not installed

Refs #22334. Thanks Tim Graham for spotting the issue.
Increased memoization cache size for language codes.

There may be more than 100 (default maxsize) commonly seen xx-yy values
on some sites. The additional memory consumption isn't significant.

Also added a comment explaining why this cache must have a maxsize.