update to groovy 4

[sgrell]

Updated everything to Groovy 4, fixes #578

The test showed showed some strange error messages:

groovy.lang.MissingPropertyException: No such property: automated for class: de.gesellix.docker.remote.api.ImageSearchResponseItem

groovy.lang.MissingPropertyException: No such property: official for class: de.gesellix.docker.remote.api.ImageSearchResponseItem

I fixed it by using the "real" getter-methods.

I don't know if this is a real issue with groovy 4 and might have to be addressed at some other places as well.

[gesellix]

I'm going to make some changes in the api-model library, so that the MissingPropertyException won't require the workaround anymore. Seems like Groovy 4 changed some semantics for property access on fields not following the JavaBeans convention (isOfficial should have been official).

[gesellix]

Thanks! I removed the commit with the workaround for the MissingPropertyException.