ci: run on macOS 11.0

Let's use the latest macOS release in CI, as users will be adopting it.

Author: indygreg

.github/workflows/pyoxidizer.yml

@@ -14,7 +14,7 @@ jobs:
           - '1.45.0'
         os:
           - 'ubuntu-20.04'
-          - 'macos-10.15'
+          - 'macos-11.0'
           - 'windows-2019'
     continue-on-error: true
     runs-on: ${{ matrix.os }}