Update antlr task + gradle

[Siedlerchr]

Fixes #6431
Fixes koppor#440

I finally found the solution.
Get rid of our AntlrPlugin and follow the idea from here:
https://stackoverflow.com/a/56029522/3450689

Update to gradle 6.6 and newer is not yet possible due to openjfx/javafx-gradle-plugin#89
Make @koppor happy :)

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[calixtus]

Except for the unnecessary tab at the eol (I like reviewdog) and the many additional linebreaks (😁), it looks good to me - although I did not work much with groovy yet.

[Siedlerchr]

I see that there is mac failing, will investigate that

[koppor]

I think, it is OK. There shouldn't be any mix of spaces and tabs.

Please fix

File contains tab characters (this is the first instance).