Apply legacy plugin last, and declare capabilities for old plugins

[jpenilla]

fixes #964 using the solution suggested by @ljacomet at https://github.com/GradleUp/shadow/issues/964#issuecomment-2345532565

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

[Goooler]

CC @ljacomet

[ljacomet]

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

[Goooler]

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