Fix Java versions to avoid crash

[madprogramer]

Rock expects java20
Opal expects java8

https://datashield.discourse.group/t/rock-2-0-and-the-transition-to-java-21/1011

[madprogramer]

Ok, the 'good pattern' is to have a Java version manager like SDKMAN!, jenv, etc. and also one for R RVM, renv, etc.

• Java Version Manager
• R Version Manager (moved to R Version Manager #24)

[madprogramer]

Working on #10 before coming back here

[madprogramer]

Fixed with ab24f08