[Unity] Ignore .gradle/ build cache

[ksmit799]

Reasons for making this change:

Newer version of Unity now use Gradle as the default build system for Android builds, thus creating a .gradle/ cache directory that sticks around after builds. This directory is merely a local cache, and serves no purpose being tracked by Git.

Links to documentation supporting these rule changes:

https://docs.gradle.org/current/userguide/directory_layout.html

[shiftkey]

Tentatively pulling this in but Gradle.gitignore might be the preferred option here (to avoid duplicating rules across templates).