Remove the support code to download JDK from Adoptium

[andsel]

With #15515 was solved a problem of version selection when downloading JDK builds from Adoptium repository. Originally Logstash's versions.yml defined the full parameters (major, minor, patch and build number) to download the JDK, but in certain circumstances a build for a specific version (es 9.1) is available only for an OS and not for all the others. To avoid to defines the exact version for each OS/arch combination the #15515 switched to download the latest version for a specific major from Elastic's JDK inventory. This left around the code to download from Adoptium, which is not anymore used, and also the full list of version parts (minor, patch and build number) which is not useful because just the major is what's needed.

• remove the old unused code
• update the version.yml to specify just the major and remove all the other useless fields
• fix the naming of the downloaded artifact, from jdk-21.0.3-darwin-aarch64.tar.gz to jdk-21-darwin-aarch64.tar.gz