feat(datasource/unity3d): Use Unity Releases API

[bdovaz]

Changes

The API has been changed to use the new Unity Releases API instead of the "old RSS system".

The new API is richer in metadata, allows more granularity by having queries (number of results, stream, os, architecture, version, ...). Example: https://services.api.unity.com/unity/editor/release/v1/releases?stream=LTS&platform=WINDOWS&architecture=X86_64

I have removed many tests that no longer make sense and were there before because of the inconsistencies of the RSS system. This new API has filters and these inconsistencies no longer occur.

I have also aligned the Unity terminology of streams (lts, tech, alpha, beta).

Context

Unity Releases API: https://services.docs.unity.com/release/v1/#tag/Release/operation/getUnityReleases

Example call: https://services.api.unity.com/unity/editor/release/v1/releases?stream=LTS

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[bdovaz]

@viceice apparently there is nothing pending, can you merge now? Thanks!