chore: bump macos runner version (#27)

GitHub Action is sunsetting the macOS 10.15 Actions runner. It will stop working intermittently until being completely removed by 2022-8-30: https://github.blog/changelog/2022-07-20-github-actions-the-macos-10-15-actions-runner-image-is-being-deprecated-and-will-be-removed-by-8-30-22

.github/workflows/ci.yml

@@ -87,7 +87,7 @@ jobs:
               compiler: clang-12,  cxxstd: '03,11,14,17,20', os: ubuntu-20.04, stdlib: libc++, install: 'clang-12 libc++-12-dev libc++abi-12-dev' }
 
           # OSX, clang
-          - { compiler: clang,     cxxstd: '03,11,14,17,2a', os: macos-10.15, sanitize: yes }
+          - { compiler: clang,     cxxstd: '03,11,14,17,2a', os: macos-11, sanitize: yes }
 
           # Coverity Scan
           # requires two github secrets in repo to activate; see ci/github/coverity.sh