Protobuf plugin uses Configuration#fileCollection(Dependency...) method

[jvandort]

Gradle plans to deprecate this method in Gradle 9.0 (See gradle/gradle #25124).

There is a single usage of one of these methods here at config.fileCollection(dep). Changing that to config.getFiles() should have the same behavior.

The files(Dependency...) method filters the contents of the configuration to only include files from the provided dependencies and it's transitive dependencies. Since there is only one dependency in the configuration, using this method is unnecessary.