Calculate core version with 'git describe' and update automatically in changelog as well

[nielsenko]

This PR updates the way we calculate the core version to allow us to use unofficial releases, and still report a unique tag, instead of just tagging with the version number of the lates official release.

It just uses git describe instead of parsing the dependencies.list file.

git describe --abbrev=5 do the right thing on pure tagged commits as well, fx.

• a5e87a39c (tag: v13.26.0) core v13.26.0

produces:

realm-core (a5e87a3) [$] via △ v3.28.2 via  v14.21.3 via 🐦 v5.8.1
❯ git describe --abbrev=5
v13.26.0