PSC Vote: Remove support for PostgreSQL 12 and PostgreSQL 13 in PostGIS 3.7

[Regina Obe]

I think it's time to say bye to support for PostgreSQL 12 and PostgreSQL 13
for master branch.

In the past we'd held on to PostgreSQL 12 cause we didn't have that many
guards (and the biggie was the change between 11 and 12 that introduced
possibility of CTEs not materialized and our utilization of function costing
magic).
But PG13 has a fair number of guards in there and PostgreSQL 12 and 13 are
no longer supported by PGDG, so it feels time to say bye bye to both.

+1

I'll give 3 days for dissenting votes before I rip support out of master.

Thanks,
Regina