Fix CI

Github moved the macos-latest runner to ARM M1. There are no precompiled
rakudo binaries available for that release though. Bump to 2021.07.
That's the first release with MacOS ARM binaries on offer.

Author: patrickbkr

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
           - macOS-latest
           - windows-latest
         raku-version:
-          - "2021.03"
+          - "2021.07"
           - "latest"
     runs-on: ${{ matrix.os }}
     steps: