Java 20 no longer supports --target 7

This effectively leads to Java 20 not being able to support --release 7 . Trying to do so fails with:
```
The project was not built due to "release 7 is not found in the system". Fix the problem, then try refreshing this project and building it since it may be inconsistent
```
as can be easily reproduced by e.g. running ModuleBuilderTests#testReleaseOption5

Start by running ModuleBuilderTests#testReleaseOption5 and inspect the release-option handling it exercises. Trace why Java 20 reports that release 7 is unavailable, then verify that the test and the described --release 7 scenario work as expected.

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