Fix the build against JDK 20 (#3323)

When using JDK 20, the following error would occur when attempting to
build or run tests:

```
Execution failed for task ':codegen-client:compileKotlin'.
> Inconsistent JVM-target compatibility detected for tasks 'compileJava' (20) and 'compileKotlin' (11).
```

This PR explicitly sets the Java source and target versions to fix this
error.

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