Update Intel Mac builds to macos-13 images for MacOS GitHub Actions

macos-12 images will be removed on 3.12.2024, and macos-14 no
longer support Intel-based MacOS in free (OSS) tier.

Author: horenmar

.github/workflows/mac-builds-m1.yml

@@ -1,4 +1,4 @@
-name: M1 Mac builds
+name: Arm Mac builds
 
 on: [push, pull_request]
 

.github/workflows/mac-builds.yml

@@ -1,10 +1,14 @@
-name: Mac builds
+name: Intel Mac builds
 
 on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: macos-12
+    # From macos-14 forward, the baseline "macos-X" image is Arm based,
+    # and not Intel based. Thus this is the newest image we can use for
+    # Intel MacOS CI, and there don't seem to be any plans to keep providing
+    # the Intel based images for free to OSS projects.
+    runs-on: macos-13
     strategy:
       matrix:
         cxx: