Update the MacOs runner in our CI

As the MacOS 12 GitHub Action runner is no longer supported, we use more
up-to-date versions in our CI. Also the version of the Ubuntu runner is
now specified to get a better control, on what is used here.

.github/workflows/ci.yml

@@ -54,7 +54,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest, macos-12, macos-14, windows-latest ]
+        os: [ ubuntu-24.04, macos-13, macos-14, macos-15, windows-latest ]
         python-version: [ '3.11' ]
     runs-on: ${{ matrix.os }}
 
@@ -112,7 +112,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest, macos-12, macos-14, windows-latest ]
+        os: [ ubuntu-24.04, macos-13, macos-14, macos-15, windows-latest ]
         python-version: [ '3.11', '3.12' , '3.13']
     runs-on: ${{ matrix.os }}