ci: replace macOS 10.15 with macOS 11 in CI workflow

GitHub started the deprecation process for macOS 10.15 runners
on 2022-05-31, and macOS 10.15 will be completely unsupported by
2022-08-30 (if things proceed as planned).

See <actions/runner-images#5583> for
more information on the removal of the macOS 10.15 images.

This issue already hit us during the build for PR boostorg#716, because
at that time there was a scheduled brownout for macOS 10.15
builds to raise awareness of the upcoming removal.

.github/workflows/ci.yml

@@ -79,7 +79,7 @@ jobs:
             os: ubuntu-20.04
           - toolset: clang
             cxxstd: "14,17"
-            os: macos-10.15
+            os: macos-11
 
     runs-on: ${{matrix.os}}