sbt-antlr4 version needs a bump

A clean checkout of the repository does not build. When executing sbt update, sbt errors and exits with

java.lang.IllegalArgumentException: requirement failed: A named attribute key must start with a lowercase letter: Build dependency required for parsing grammars, scoped to plugin.

I traced this back to an issue with sbt-antlr4. The version reference in project/sbt-antlr4.sbt should be bumped from 0.8.0 to 0.8.1 to fix this.

This is also causing problems with CI, e.g. https://gitlab.anu.edu.au/mu/mu-client-pypy/-/jobs/20261