Follow up on migration from ANTLR 4 to ANTLR 5 on code and infrastructure level

[ftomassetti]

This combines three very small changes connected to the migration to ANTLR 5:

• Fix #21: Remove reference to Python in GitHub Actions workflow
• Fix #22: Rename Antlr4MojoTest in Antlr5TMojoTest
• Remove reference to Swift target in the clean task

This PR has been extracted from #23