SpringBoot Upgrade Java 15 pipeline to use GA builds

Hi,

this PR upgrades the Java 15 pipeline to make use of GA builds.

Cheers, Christoph

Comment From: snicoll

This is strange. I ran that command with the same change as you this morning and didn’t get any result. Have you tried to run it manually?

Comment From: dreis2211

I called the API manually, yes. https://api.adoptopenjdk.net/v3/info/available_releases for example listed JDK 15 only a couple of hours ago. So I guess you were a little bit too early with your try. E.g. around 9 AM it wasn't listed yet.

Comment From: aahlenst

We published AdoptOpenJDK 15+36 around 09:30+00:00, so very late in the morning for western Europe.