os-zoo CI: Replace macos-12 run with macos-15

macos-12 runners will be removed in December.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Hugo Landau <hlandau@devever.net>
(Merged from https://github.com/openssl/openssl/pull/25715)

(cherry picked from commit 6a3d5b6e62)

.github/workflows/os-zoo.yml

@@ -105,7 +105,7 @@ jobs:
       fail-fast: false
       matrix:
         branch: [openssl-3.0, openssl-3.1, master]
-        os: [macos-12, macos-13, macos-14]
+        os: [macos-13, macos-14, macos-15]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4