Use c++23 in macos-15 GitHub Actions.

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.

Merge pull request #94 from Flamefire/test-link-update

Update Link to regression test matrix in README

Use Azure mirrors of Ubuntu .deb repositories in containers.

This reduces the likelihood of spurious CI failures caused by DDoS filters
being triggered by massive numbers of concurrent CI jobs.

Added a test for filter_iterator converting constructor.

Revert "Added a workaround for the missing MPL includes in Range."

This reverts commit dfb1fd9.

The missing includes were added to Boost.Range, the workaround is
no longer needed.

Work around compilation errors on clang-10 through 12.

Work around compilation errors on clang-10 through 12.

Updated deprecated include in docs.

Closes #83.

Updated deprecated include in docs.

Closes #83.