Removed macos-13 GitHub Actions CI job.

The macos-13 image is deprecated by GitHub Actions and will be removed.

Refs actions/runner-images#13046.

Author: Lastique

.github/workflows/ci.yml

@@ -293,9 +293,6 @@ jobs:
               - libc++-18-dev
               - libc++abi-18-dev
 
-          - toolset: clang
-            cxxstd: "11,14,17,20,2b"
-            os: macos-13
           - toolset: clang
             cxxstd: "11,14,17,20,2b"
             os: macos-14