ci: use macos-latest image for CI builds

The macos-10.15 image for CI builds is deprecated on GitHub Actions. We
should update this to a supported image.

Given we are actually running our release builds on macos-latest, we
should probably be running CI on the same image too!

Author: mjcheetham

.github/workflows/continuous-integration.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04, windows-2019, macos-10.15]
+        os: [ubuntu-18.04, ubuntu-20.04, windows-2019, macos-latest]
 
     steps:
     - uses: actions/checkout@v3