CI: use macOS 12 (instead of 13/14/latest)

The macOS jobs started failing for no reason so maybe it's
because macOS-latest now maps to macOS14 (which has a
different CPU, AFAIU) or macOS13. See:
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

.github/workflows/CI.yml

@@ -9,7 +9,7 @@ on: [push, pull_request, workflow_dispatch]
 
 jobs:
   macOS--dotnet6-and-mono:
-    runs-on: macOS-latest
+    runs-on: macOS-12
     steps:
     - uses: actions/checkout@v1
       with:
@@ -42,7 +42,7 @@ jobs:
         find . -type f -name "*.fsx" | xargs -t -I {} dotnet fsxc {}
 
   macOS--mono-only:
-    runs-on: macOS-latest
+    runs-on: macOS-12
     steps:
     - uses: actions/checkout@v1
       with:
@@ -65,7 +65,7 @@ jobs:
       run: make update-servers
 
   macOS--dotnet6-only:
-    runs-on: macOS-latest
+    runs-on: macOS-12
     steps:
     - uses: actions/checkout@v1
       with: