Dashboard and Linkage Checker to take option to resolve dependency graphs with Gradle's way

Dashboard (and Linkage Checker's ClassPathBuilder) to take an option to resolve dependency graphs with Gradle's way.

While reading errors in https://github.com/GoogleCloudPlatform/cloud-opensource-java/pull/1982#issuecomment-812277520, I noticed that the dependency conflicts (due to picking up old library versions) do not occur to Gradle users.

# Where is this affected?

The `LtsCompatibilityTestRunner` resolves the dependencies of the BOM:

```
ImmutableList bomManagedDependencies = bom.getManagedDependencies();
ClassPathBuilder classPathBuilder = new ClassPathBuilder();
ClassPathResult resolvedDependencies = classPathBuilder.resolve(bomManagedDependencies, false);
```

This resolves the dependencies in Maven's closest-win strategy. This strategy is prone to getting old dependencies which cause dependency conflicts. For Gradle users, the resolution is different; it uses highest-win strategy. Therefore, the current result shown in the pull request contain false positives and false negatives.

# GradleDependencyMediation?

We already have `MavenDependencyMediation`. I think the implementation would be creating GradleDependencyMediation.