Switch to reusable CI flow #249

We've had to periodically update our CI configs as old Github Actions test runners are deprecated. The latest is macos-13, currently causing our CI jobs to fail. Fortunately Boost's build experts have a solution: a reusable config that can be used straight out of the Boost-CI repo, as described here. We should switch to it to save on development effort and gain other benefits, like code coverage and static analysis.