fix(jdk21-preview): correct JAVA_MAJOR_VERSION extraction for the EA release link

[lemeurherve]

This PR fixes the preview (EA) release download link in the Dockerfile of the debian_jdk_21_preview image.

Related: #537

Testing done

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[dduportal]

Wrong link:
https://github.com/adoptium/temurin21.0.1-binaries/releases/download/jdk-21.0.1%2B12-ea-beta/OpenJDK21.0.1U-jdk_s390x_linux_hotspot_ea_21-0-1-12.tar.gz

Correct link:
https://github.com/adoptium/temurin21-binaries/releases/download/jdk-21.0.1%2B12-ea-beta/OpenJDK21U-jdk_s390x_linux_hotspot_ea_21-0-1-12.tar.gz

• Either you need to hardcode the 2nd occurence of JAVA_MAJOR_VERSION and remove all occurences (and the definition)
• Or eventually use my suggestion by fixing the JAVA_MAJOR_VERSION definition

[lemeurherve]

Hum... It failed on the primary branch 🤔

https://github.com/jenkinsci/docker-agent/actions/runs/6624130432/job/17992491585

ERROR: something went wrong in target "setJDK21PreviewVersionDockerBake" : "command error on refs/heads/updatecli_39548d6bb0215e91203e5365849207b3faf5504376b23ab629f00709c4208dd3: refusing to allow a GitHub App to create or update workflow .github/workflows/release-drafter.yml without workflows permission"

[lemeurherve]

Ran the action again without any issue 🤷