Apply legacy plugin last, and declare capabilities for old plugins

[jpenilla]

fixes #964 using the solution suggested by @ljacomet at #964 (comment)

Tested to fix the issue (bring behavior back to how it was before 8.3.1/with the old plugin), and to give an error when both the current and old plugin are on the classpath.

Something else that came to mind regarding the legacy plugin is that if someone puts the new plugin on the classpath, but then applies the legacy plugin id, nothing will happen, which could be confusing. But resolving that is likely better addressed in a separate PR (if at all).

> Could not resolve all files for configuration ':build-logic:compileClasspath'.
   > Could not resolve com.gradleup.shadow:shadow-gradle-plugin:8.3.3-SNAPSHOT.
     Required by:
         project :build-logic
      > Module 'com.gradleup.shadow:shadow-gradle-plugin' has been rejected:
           Cannot select module with conflict on capability 'com.github.johnrengelman:shadow:8.3.3-SNAPSHOT' also provided by [com.github.johnrengelman:shadow:8.1.1(apiElements)]
   > Could not resolve com.github.johnrengelman:shadow:8.1.1.
     Required by:
         project :build-logic
      > Module 'com.github.johnrengelman:shadow' has been rejected:
           Cannot select module with conflict on capability 'com.github.johnrengelman:shadow:8.1.1' also provided by [com.gradleup.shadow:shadow-gradle-plugin:8.3.3-SNAPSHOT(apiElements)]

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• CHANGELOG's "Unreleased" section has been updated, if applicable.

[Goooler]

CC @ljacomet

[ljacomet]

Sorry, I said I would attempt this and in the end did not find the time.

This LGTM!

The one thing I wondered was about the capability addition: should it be done in a patch release or in a minor?
My preference would be to do it in a minor as it might end up breaking builds (for good reasons) while the fix to the order of application fits perfectly a patch release.

[ljacomet]

And thank you @jpenilla for taking care of fixing my mistake!

[Goooler]

I'm planning to release this in 8.3.4, WDYT?