[github-actions] update to macos-12 (#7930)

macos-10.15 is deprecated.

.github/workflows/build.yml

@@ -303,7 +303,7 @@ jobs:
             CXX: clang++
           - CC: gcc
             CXX: g++
-    runs-on: macos-10.15
+    runs-on: macos-12
     env:
       CC: ${{ matrix.CC }}
       CXX: ${{ matrix.CXX }}

.github/workflows/posix.yml

@@ -226,7 +226,7 @@ jobs:
 
   pty-macos:
     name: pty-macos OT_DAEMON=${{ matrix.OT_DAEMON }}
-    runs-on: macos-10.15
+    runs-on: macos-12
     strategy:
       fail-fast: false
       matrix: