[CI testing] Build fails because Python2 is no longer supported on Github Actions.

[kaby76]

From the build:

 Warning: The support for python 2.7 will be removed on June 19. Related issue: https://github.com/actions/setup-python/issues/672
  Version 2.x was not found in the local cache
  Error: The version '2.x' with architecture 'x64' was not found for macOS 11.7.7.
  The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Yes, what day is it today?? June 21.

We can't test the Python2 Antlr target anymore. We really should not be trying to support Python 2 with Antlr. Drop the target.

[ericvergnaud]

Agreed, now's the time

[KvanTTT]

It's duplicate: #3266

[kaby76]

It's not a duplicate. The build is failing because Python2 is not available on Github Actions. People can still use Python2 and Antlr if they want, even for 4.13.0. Just last week, this was committed. It looks like it can't be tested in Github anymore. I'm changing the title to be more precise. If folks want to support it still, keep on modifying the code, but just fix the build so other changes (like the one I need for Dart) don't get flagged as failing all the time.