bug#43275: Upgrade antlr-mode to latest version

[Wedler, Christoph]

  >> The antlr-mode shipped with Emacs is version 2.2c, but there is a
  >> version 3.1.5 available at:

  >>     https://sourceforge.net/projects/antlr-mode/files/

  >> Could we upgrade it to the latest version?

> I hope we can, but the fact that updating has not occurred all along
> indicates a lack of communication with the developer of antlr-mode.

If you need anything from my side (I am the author of antlr-mode),
please let me know.
Unfortunately, it is unrealistic for me to change the Emacs code base directly
(due to inexperience in it & time constraints).

I use antlr-3.1.5 myself on Emacs 26.3 with ANTLR v4.8/JS,
so there is hopefully no bigger adaption necessary...

Best regards,
Christoph