explicitly install Gradle JDK
explicitly install Gradle JDK

It seems like for the next Gradle version even the "update wrapper" task does some analysis of the Java bytecode version. The symptom now is something like

```
/usr/bin/gradle wrapper --gradle-version 6.8.2 --distribution-type bin --gradle-distribution-sha256-sum 8de6efc274ab52332a9c820366dd5cf5fc9d35ec7078fd70c8ec6913431ee610
...
  > Configure project :
  Configured JDK: 1.8

  FAILURE: Build failed with an exception.

  * What went wrong:
  java.lang.UnsupportedClassVersionError: org/javamodularity/moduleplugin/ModuleSystemPlugin has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
  > org/javamodularity/moduleplugin/ModuleSystemPlugin has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
```

Thus I've added an explicit install step to provide a JDK that is new enough. The concrete version should be irrelevant, since we won't execute a real build with this JDK, but only verify the wrapper.

Signed-off-by: Peter Gafert <cdbfcea01cf62f959f8b10da948a53f2869d9b6c@tngtech.com>

TNG/ArchUnit,TNG/ArchUnit