GEOS 3.6 and GEOS 3.7 drop for PostGIS 3.5.0

[robe]

As noted in PostGIS Dev mailing list

​https://lists.osgeo.org/pipermail/postgis-devel/2023-October/030147.html

​https://lists.osgeo.org/pipermail/postgis-devel/2023-October/030149.html The plan is to drop GEOS 3.6 and GEOS 3.7. 3 + votes and no - votes.

As part of that, the following needs to happen

• Revise configure to not allow building for GEOS < 3.8
• Update doc references in postgis.xml (and any hard-coded in docs)
• Remove all ifdefs in code and tests for GEOS 3.6 and 3.7
• Stop ci testing on 3.6 and 3.7
  • Woodie/Dronie - I've already upped the GEOS from 3.6 to 3.10 and looks like build passed.
  • Remove 3.7 testing from Github Actions
    • The 3.7 matrix is the only one testing proj6 and pg12, so we should probably replace with slightly newer that has all but geos38 instead of 37