Document deprecation rationale in DefaultMaven

[anukalp2804]

Summary:
This pull request documents the rationale for a deprecated method in DefaultMaven.

Why:
The existing JavaDoc did not explain why the method is deprecated or clarify that
it is intended for internal Maven Core use only, which could confuse external users.

What:

• Replaced informal JavaDoc with a clear explanation of the deprecation rationale
• Clarified intended usage boundaries for the deprecated method

Scope:

• Documentation-only change in Maven Core
• No behavioral or API changes

Testing:

• mvn -DskipTests install (BUILD SUCCESS)

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[desruisseaux]

Git history tells me that it was deprecated on November 21, 2023:

git show e89b6fd -- maven-core/src/main/java/org/apache/maven/DefaultMaven.java

I don't know which Maven version was released after that time. Maybe the "release" page of Maven web site can tell that.

[desruisseaux]

Actually an easier way to find the release is git tag --contains e89b6fd. Deprecation was in maven-4.0.0-alpha-9, but it is probably not worth to go to that level of details. I think that 4.0.0 is sufficient.

[anukalp2804]

Actually an easier way to find the release is git tag --contains e89b6fd. Deprecation was in maven-4.0.0-alpha-9, but it is probably not worth to go to that level of details. I think that 4.0.0 is sufficient.

Thank you for checking the history and confirming the version.
I have updated the code to use @deprecated(since = "4.0.0") and removed
the redundant Javadoc deprecation tag, keeping only the rationale text.

Please let me know if anything else should be adjusted.

[github-actions]

@desruisseaux Please assign appropriate label to PR according to the type of change.

[anukalp2804]

Thanks for the review and merge!