[PostGIS] #6024: Remove support for PostgreSQL 12 and 13 in PostGIS 3.7

[PostGIS]

#6024: Remove support for PostgreSQL 12 and 13 in PostGIS 3.7
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 Reporter:  robe     |      Owner:  robe
     Type:  defect   |     Status:  assigned
 Priority:  blocker  |  Milestone:  PostGIS 3.7.0
Component:  build    |    Version:  master
 Keywords:           |
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 As agreed in

 https://lists.osgeo.org/pipermail/postgis-devel/2025-November/030681.html

 We should drop support for PostgreSQL 12 and 13 and remove all conditional
 code related to those.  Both have reached EOL and are no longer packaged
 by most packagers.
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/6024>
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