Upgrade CI configuration following macOS-11 deprecation ...

macOS-11 is going away at the end of June.
As a consequence, we upgrade to macOS-12, upgrade the supported
frontend versions, and add some arm64 tests to prepare for
future upgrades.

Trivial: Remove an extra whitespace in CI file

Add a CI step to upload assets on release creation ...

We haven't had assets generated for releases since v1.29.0,
however some external code depend on them (dlang-community/setup-dlang, namely).
This will fix it and make it less likely to be forgotten in the future.