Tidy up test migration and fix Gradle 9 test compatibility

[FinlayRJW]

Important

Depends on #1506 and #1538 being merged first.

Before this PR

There were some rough edges left over from the test migration that needed tidying up, and ConfigurationOnDemandTest had a test that fails on Gradle 9.

The transitive_dependencies_cause_upstream_projects_to_be_configured_sufficiently_early test resolved another project's configuration directly from a task action:

tasks.register('writeClasspathOfA') {
    doLast {
        println project(':projectA').configurations.runtimeClasspath.files
    }
}

This cross-project configuration resolution is no longer allowed in Gradle 9:

Execution failed for task ':projectC:writeClasspathOfA'.
> Resolution of the configuration ':projectA:runtimeClasspath' was attempted without an exclusive lock.
  This is unsafe and not allowed.

After this PR

==COMMIT_MSG==
Fix Gradle 9 compatibility in ConfigurationOnDemandTest and tidy up the test migration
==COMMIT_MSG==

The writeClasspathOfA task is removed. Instead, the test uses the existing writeClasspath task on projectC (registered for all subprojects in @BeforeEach), which resolves projectC's own runtimeClasspath. Since projectC -> projectB -> projectA, the transitive dependency transitive-test-dep is still present and the version constraint from the lock file (1.1.0 rather than 1.0.0) is still validated.

Other tidy-ups:

• Remove direct nebula-test dependency (no longer needed after migration)
• Add Gradle 7 and 9 to the test matrix in gradle-test-versions.yml
• Remove stale @AdditionallyRunWithGradle("9.3.0") from CheckUnusedConstraintIntegrationTest (covered by the test matrix now)
• Fix file:/// URI to uri() in VersionsPropsPluginIntegrationTest see Add explicit callout to use uri("%s") instead of raw file:/// URLs gradle-plugin-testing#416 for reasoning

Possible downsides?

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• Fix (Fixing an issue with existing functionality)
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• Migration (Automatically moving data/functionality to a new system)

Description

Fix Gradle 9 compatibility in ConfigurationOnDemandTest and tidy up the test migration

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💡 Improvements

• Fix Gradle 9 compatibility in ConfigurationOnDemandTest and tidy up the test migration (#1630)