GHA: Update macOS runners

Remove macos-12, not supported anymore.
Add macos-15, runner is available in Beta status.

Change-Id: Ibaa591904095a824df2c0c5aaff73687e0af24fe
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Acked-by: Yuriy Darnobyt <yura.uddr@gmail.com>
Message-Id: <20241015205627.25644-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29551.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6f74d39)

.github/workflows/build.yaml

@@ -194,16 +194,16 @@ jobs:
       matrix:
         ssllib: [ openssl11, openssl3, libressl]
         build: [ normal, asan ]
-        os: [macos-12, macos-13, macos-14]
+        os: [macos-13, macos-14, macos-15]
         include:
            # macos14 and newer runners use ARM CPUs and homebrew uses /opt/homebrew/
            # on ARM instead of /usr/local/
-          - os: macos-12
-            homebrew: /usr/local/opt
           - os: macos-13
             homebrew: /usr/local/opt
           - os: macos-14
             homebrew: /opt/homebrew/opt
+          - os: macos-15
+            homebrew: /opt/homebrew/opt
           - build: asan
             cflags: "-fsanitize=address -fno-optimize-sibling-calls -fsanitize-address-use-after-scope -fno-omit-frame-pointer -g -O1"
             ldflags: -fsanitize=address