CI: Remove macos-10.15, it has been deprecated

Author: danmar

.github/workflows/CI-unixish.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         compiler: [clang++, g++]
-        os: [ubuntu-18.04, ubuntu-20.04, macos-10.15]
+        os: [ubuntu-18.04, ubuntu-20.04]
       fail-fast: false
 
     runs-on: ${{ matrix.os }}