RFR: Update GHA to jdk20 [v5]

[Jorn Vernee]

On Mon, 12 Dec 2022 13:53:42 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Maurizio Cimadamore has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:
>> 
>>   Drop macos
>
> .github/workflows/test.yml line 61:
> 
>> 59:         ${{ matrix.JAVA20_HOME }}/bin/java --version
>> 60: 
>> 61:     - name: 'Setup Java 19'
> 
> Not sure why this is/was here?

We need 2 java versions, one to run Gradle, and one  to build/run jextract. Version 18 was needed to run gradle.

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PR: https://git.openjdk.org/jextract/pull/96