remove MacOS 10.15 from testing matrix

This version is not supported, anymore. More info at
actions/runner-images#5583

Signed-off-by: Max Jonas Werner <[email protected]>

Author: makkes

.github/workflows/tests.yaml

@@ -63,7 +63,7 @@ jobs:
   darwin-amd64:
     strategy:
       matrix:
-        os: [macos-10.15, macos-11, macos-12]
+        os: [macos-11, macos-12]
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: