Remove unreleasable platforms from JDK 20 pipeline config

[jiekang]

The AIX, s390x, arm32, and ppc64le platforms can't be released in JDK 20 due to compliance issues. We can remove them from the pipeline config so they don't take resources during a release that can be used for jdk 8/11/17, and also have less noise for triage.

From April retro:

          JDK20, we still launched the builds for the 3 platforms we knew were not in plan for the release (AIX, s390x, arm32) and I caught myself triaging one, before I remembered that I did not need to put effort there.

Originally posted by @smlambert in adoptium/adoptium#225 (comment)

[sxa]

JDK20 is now EOL - closing.