Replace macOS 10.15 runners with macOS 11 ones

10.15 is being phased out by GitHub, so switch to the still supported
runner platform version.

Closes #22698.

.github/workflows/ci_mac.yml

@@ -93,12 +93,12 @@ jobs:
           runner: self-hosted
           arch: arm64
           configure_flags: --with-cxx=14 --enable-universal_binary=arm64,x86_64 --disable-shared --disable-debug --enable-optimise
-        - name: wxMac macOS 10.15
-          runner: macos-10.15
+        - name: wxMac macOS 11
+          runner: macos-11
           arch: x86_64
           configure_flags: --disable-sys-libs
         - name: wxiOS
-          runner: macos-10.15
+          runner: macos-11
           arch: x86_64
           configure_flags: --with-osx_iphone --enable-monolithic  --disable-sys-libs --host=i686-apple-darwin_sim --build=x86_64-apple-darwin17.7.0
           xcode_sdk: iphonesimulator