Drop macOS 12 support, add macOS 15 instead

The macOS 12 actions runner has been deprecated on 2024-10-07 and is
planned for complete removal on 2024-12-03 [1]. I believe it's about
time to stop using macOS 12, and instead cover macOS 15 (beta) instead.

[1]: actions/runner-images#10721

Author: ikalnytskyi

.github/workflows/ci.yml

@@ -21,9 +21,9 @@ jobs:
           - ubuntu-20.04
           - ubuntu-22.04
           - ubuntu-24.04
-          - macos-12
           - macos-13
           - macos-14
+          - macos-15
           - windows-2019
           - windows-2022
     steps:
@@ -59,9 +59,9 @@ jobs:
           - ubuntu-20.04
           - ubuntu-22.04
           - ubuntu-24.04
-          - macos-12
           - macos-13
           - macos-14
+          - macos-15
           - windows-2019
           - windows-2022
         postgres-version: