Gradle Module Metadata - publish by default

[jjohannes]

Publish Gradle Module Metadata by default, which means enabling all behavior that is now hidden behind the GRADLE_METADATA preview flag. The flag should then be made inactive.

[jjohannes]

@ljacomet @melix I have merged #10274 already. Please have a look at the PR for more details and let me know if you have any comments.

I assume that this change will break some builds that work with custom repositories without proper metadata with a message like this:

> Could not find org:foo:1.0.
     Searched in the following locations:
       - https://repo.myrepo.org/repo/org/foo/1.0/foo-1.0.pom

I was wondering if we should extend the message with something like:

If the artifact you are trying to retrieve is located at the given position without metadata in 'pom' format, you need to adjust the 'metadataSources {}' (link to docs) of the 'https://repo.myrepo.org/repo/' repository declaration.

[jjohannes]

Looking into aritfact and classifier handling (traditional vs. GMM-based), I noticed a few more inconsistencies and found a small bug resulting in these changes:

• Do not drop variant attributes in results based on maven artifacts 389c12f
• Realized component variants need to provide all attributes b79c2a9
• Align implementations of artifact identifier display names dbe5014
• Fix AttributeConfigurationSelector to return the filtered result 3255f1d