Refactor the plugin

[Azurelol]

Description

Remove support for versions less than 1.2 of the Unified Build System. Redesign the tasks, gping down from 4 unity-related tasks to 2. The secret handling has also been removed from the plugin.

Changes

• UnityBuildPlayerTask, UnityBuildRequestTask
• Layout of the UnityBuildPlugin
• Base task is now BuildUnityTask, no longer abstract.
• Default task to build a player is now the PlayerBuildUnityTask
• Moved UBSVersion to the models package.
• Secret handling tasks and procedures from the plugin (because they are handled by the atlas-secrets plugin)

[Azurelol]

This plugin will be archived, replaced by atlas-unified-build-system.