Unsupported class file major version 69

I can't compile this repo:

```
eclipse-format$ ./gradlew shadowJar

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring root project 'eclipse-format'.
> Could not open cp_proj generic class cache for build file 'build.gradle'.
   > BUG! exception in phase 'semantic analysis' in source unit '_BuildScript_' Unsupported class file major version 69
```