Update R8 and then make a new release of Gr8

[sschuberth]

          It should be released by now. What we need to do to update R8 in here and then make a new release of Gr8 and then use that new Gr8 release in `main` and then we can remove the limitation. Bootstrap all the way down 😅 

Note that in this specific case, toolchains wouldn't help anyways because R8 takes the runtime libs from the current JRE (unless we used a separate JavaExec task to run R8 itself but there are so many hours in a day sadly).

Originally posted by @martinbonnin in #22 (comment)