Remove support for PostgreSQL < 12.

[dwsteele]

These versions are EOL so it make sense to remove them.

[keithf4]

Compiled fine with PG16

Tested with PG11 and it spit out a bunch of errors, but the first one was

keith@dell-laptop:~/Repos/set_user-dsteele$ make install
cat extension/set_user.sql > extension/set_user--4.0.1.sql
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -O2 -fPIC -I. -I./ -I/opt/pgsql1121/include/server -I/opt/pgsql1121/include/internal  -D_GNU_SOURCE   -c -o src/set_user.o src/set_user.c
In file included from src/set_user.c:57:
src/compatibility.h:171:2: error: #error "This extension only builds with PostgreSQL 12 or later"
  171 | #error "This extension only builds with PostgreSQL 12 or later"
      |  ^~~~~
src/compatibility.h:175:1: warning: data definition has no type or storage class
  175 | _PU_HOOK;
      | ^~~~~~~~
src/compatibility.h:175:1: warning: type defaults to ‘int’ in declaration of ‘_PU_HOOK’ [-Wimplicit-int]
[...]

[dwsteele]

@keithf4 I do wish I could make that error a little cleaner but not sure how it is possible unless -Wfatal-errors is set and that is up to the user. Anyway, it works.

Thanks for the review!

[dwsteele]

I had to recommit this as 1ff48cf since tests failed due to the CI container removing docker compose.