Fix and improve the docs about Publishing Java packages with Gradle

[mahozad]

• Merge a somewhat duplicate sentence about setting environment variables in the deploy step
• Fix an error in a repository name (Maven Central instead of GitHub Packages) (may have happened because of a copy-paste)
• Fix another paragraph that only referred to configuration of GitHub Packages while it was about configuring both Maven Central and GitHub Packages (may have happened because of a copy-paste)

[welcome]

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[HitMalc]

Cool

[HitMalc]

Okay