Update dependency openjdk/jdk to v21

[renovate]

This PR contains the following updates:

Package	Update	Change
openjdk/jdk	major	20 → 21

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Release Notes

openjdk/jdk (openjdk/jdk)

v21

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[wmdietl]

I re-ran these tests and they still fail with:

Warning: Checksum not available for: https://download.java.net/java/early_access/jdk27/21/GPL/openjdk-27-ea+21_linux-x64_bin.tar.gz
Error: Checksum verification failed, deleting cached archive
Error: Exception in thread "main" java.lang.AssertionError: Checksum verification failed, deleting cached archive
	at Download$Downloader.verifyChecksums(Download.java:214)
	at Download.main(Download.java:101)
	at Download.main(Download.java:42)
Error: Process completed with exit code 1.

[wmdietl]

The v21 tag was added here https://github.com/openjdk/jdk/tags
but the latest EA build is currently still v20 https://jdk.java.net/27/
So this might still need a few more days to have a build.