RFR: Update GHA to jdk20

[Maurizio Cimadamore]

On Mon, 12 Dec 2022 13:53:16 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This PR updates the github actions on the jdk20 branch

.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?

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