Stop testing on PG12 (out of support)

(cherry picked from commit 3cbdba4)

Author: roji

.github/workflows/build.yml

@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-22.04, windows-2022]
-        pg_major: [17, 16, 15, 14, 13, 12]
+        pg_major: [17, 16, 15, 14, 13]
         config: [Release]
         include:
           - os: ubuntu-22.04