CI should test BSC with newer GHC versions

[quark17]

PR #580 will upgrade the CI to use GHC 9.2.8, which is a stable recommended version, and what we will use for pre-built releases. There are, however, newer versions of GHC, specifically 9.4.5 and 9.6.2. We should add jobs to the CI to test BSC with these newer versions as well, so that we can monitor compatibility (and not discover it later when we want to upgrade).

[quark17]

I have a prototype on the reorg-ci branch of my fork, which abstracts the build and test jobs into a call-able workflow, that the top-level CI can call with specific OS and GHC version inputs. GitHub's display of nested jobs is a little confusing (and different for matrix and non-matrix callers), which you can see in the Actions tab, for example this run.

[quark17]

FYI, callable (or reusable) workflows is documented here: https://docs.github.com/en/actions/using-workflows/reusing-workflows

And I notice that you can specify a subset of a matrix to run, by not specifying any matrix variables and using include to specify only the configurations to run (see example in the GitHub documentation on matrix). Currently, I've defined two separate top-level jobs (one for the OS matrix and one for the latest GHC), but we could just combine those into one job that explicitly lists all the combinations to run.