[MDEP-971] Remove deprecated dependency:sources goal

[wilx]

Summary

• remove the deprecated dependency:sources goal and its OldResolveDependencySourcesMojo implementation
• remove the obsolete goal from the site index
• update skip coverage and the legacy MDEP-66 harness to use dependency:resolve-sources

The generated plugin descriptor now exposes resolve-sources without the removed sources alias.

Fixes #1481.

Compatibility

The project supports Maven 3.6.3 and Java 8. This change removes an entry point and introduces no new Maven APIs, constructors, methods, or runtime features.

Testing

• mvn spotless:apply
• mvn clean verify
• mvn clean verify -Prun-its on JDK 21: 435 unit tests passed with 1 existing skip; all 102 Invoker builds passed
• inspected target/classes/META-INF/maven/plugin.xml to confirm that it contains resolve-sources and not sources

Following this checklist to help us incorporate your
contribution quickly and easily:

• Your pull request should address just one issue, without pulling in other changes.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body.
  Note that commits might be squashed by a maintainer on merge.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied.
  This may not always be possible but is a best-practice.
  No negative test was added solely to assert the absence of the removed goal; existing skip coverage was moved to resolve-sources.
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