[Backport maven-4.0.x] Fix #12931: do not rewrite unmodified POMs in mvnup apply

[gnodet]

Backport of #12938 to maven-4.0.x.

Summary

mvnup apply always logged Saving modified POMs... and wrote every discovered POM, even when every strategy reported 0 POM(s) modified. That both misleads the user and rewrites files that did not change.

UpgradeResult already tracks modifiedPoms. Apply now:

• writes only those POMs
• skips the save report entirely when the set is empty

Fixes #12931

[gnodet]

✅ Looks Good

Clean backport of #12938 (merged to master) to maven-4.0.x. The diff is byte-for-byte identical to the original.

Technical notes:

• The change from returning int to UpgradeResult in doUpgrade correctly exposes modifiedPoms() to the caller without breaking Apply or Check subclasses.
• The exception-handling path constructs UpgradeResult.failure() with empty modifiedPoms, preventing saveModifications from being called on failure — correct behavior.
• Tests use FileTime comparison to verify unmodified POMs are not rewritten (not just that the method isn't called) — robust assertion strategy.
• No backport to 3.x branches needed — mvnup is a Maven 4 feature.

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