ci: remove macos-10.15 runner (affected by a brownout, and scheduled to be removed in 12/1/2022)

See: actions/runner-images#5583
The brownouts for this runner image, will get increasingly more frequent.

Author: spytheman

.github/workflows/native_backend_tests.yml

@@ -53,7 +53,7 @@ jobs:
       matrix:
         os: [
           ubuntu-18.04, ubuntu-20.04,
-          macos-10.15, macos-11, macos-12,
+          macos-11, macos-12,
           windows-2019, windows-2022
         ]
     runs-on: ${{ matrix.os }}