Add test for ChangeParentPom retaining managed version from grandparent

[timtebeek]

Context

• In Maven add missing version to a dependency #6770 it was reported that ChangeParentPom would not add an explicit version for spring-retry when upgrading spring-boot-starter-parent from 3.5.9 to 4.0.2, since Spring Boot 4 no longer manages spring-retry.

What this PR does

Adds a test bringsDownRemovedManagedVersionFromGrandparent that verifies ChangeParentPom correctly retains managed versions even when the dependency management comes through a grandparent POM (e.g. spring-boot-starter-parent → spring-boot-dependencies).

The test confirms that ChangeParentPom does correctly:

1. Detect that spring-retry is managed by spring-boot-starter-parent:3.5.9 (via spring-boot-dependencies:3.5.9)
2. Detect that spring-boot-starter-parent:4.0.2 no longer manages spring-retry
3. Add spring-retry with explicit version to <dependencyManagement> in the child POM

This is a regression test complementing the existing bringsDownRemovedManagedVersion test which only covers the single-level parent case (spring-boot-dependencies directly).

[timtebeek]

The context here is that a community user had reported an issue, but we've not yet been able to replicate it; we can keep this open for a short while to make it easier to suggest updates to the test, or otherwise close the PR again.