Use MacOS 11 instead of 10.15 as 10.15 is depreciated

 - see actions/runner-images#5583

Author: drojf

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         steam: ["-steam", ""]
-        os: [windows-2019, macos-10.15, ubuntu-20.04]
+        os: [windows-2019, macos-11, ubuntu-20.04]
         isinclude: [false] # Makes include add new jobs instead of editing existing ones
         include:
           - os: ubuntu-20.04