Skip PostgreSQL 12 by default for CICD

TimescaleDB 2.13.0 does not support PostgreSQL 12 anymore. This
currently causes our CI to fail. To still be able to test PostgreSQL 12,
we will run it if the TimescaleDB tag is set to 2.11.2

https://docs.timescale.com/about/latest/release-notes/#timescaledb-2120-on-2023-09-27

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
       fail-fast: false
       max-parallel: 12
       matrix:
-        pgversion: [12, 13, 14, 15, 16]
+        pgversion: [13, 14, 15, 16]
         container:
         - os: rockylinux
           version: "9"
@@ -80,6 +80,17 @@ jobs:
             os: centos
             version: "7"
           pgversion: 16
+        # TimescaleDB as of 2.12.0 no longer supports PostgreSQL 12.
+        # To allow us to do run CI against PostgreSQL 12, we therefore explicitly
+        # remove all CI's, except if we are run against the latest supported TimescaleDB
+        # version for PostgreSQL 12, which is 2.11.2
+        include:
+        - pgversion: 12
+          container:
+            image: debian-11-amd64
+            os: debian
+            version: "11" 
+          tsdb_commit: 2.11.2
     env:
       # TODO Why?  Cargo default is to pass `-C incremental` to rustc; why don't we want that?
       #   https://doc.rust-lang.org/rustc/codegen-options/index.html#incremental
@@ -113,7 +124,7 @@ jobs:
 
     - name: Build and install TimescaleDB
       if: ${{ (github.event_name == 'schedule' && github.event.schedule == '0 8 * * 1-4') || inputs.tsdb-commit != '' }}
-      run: ./tools/install-timescaledb '${{ matrix.pgversion }}' '${{ inputs.tsdb-repo || 'https://github.com/timescale/timescaledb.git' }}' '${{ inputs.tsdb-commit == '' && 'main' || inputs.tsdb-commit }}'
+      run: ./tools/install-timescaledb '${{ matrix.pgversion }}' '${{ matrix.tsdb_commit || inputs.tsdb-repo || 'https://github.com/timescale/timescaledb.git' }}' '${{ inputs.tsdb-commit == '' && 'main' || matrix.tsdb_commit || inputs.tsdb-commit }}'
 
     # TODO After the container image contains a primed target dir, is this still worth it?
     #   Only possible advantage is this one is per-pg-version but what's the impact?