feat: github action prep for JDK21

[SimonVerhoeven]

What's changed?

Set the min GitHub java version to 17 in the java 17 recipe, and use the default value in 21 for when the recipe is updated in rewrite-github-actions

What's your motivation?

Preparation for Java 21

Anything in particular you'd like reviewers to focus on?

Anyone you would like to review specifically?

Have you considered any alternatives or workarounds?

Any additional context

Checklist

• I've added unit tests to cover both positive and negative cases
• I've added the license header to any new files through ./gradlew licenseFormat
• I've used the IntelliJ IDEA auto-formatter on affected files

[knutwannheden]

@SimonVerhoeven Thank you!