[FAQ] Update info about the oldest Unity version supported

[iulianatodoran]

Description

Currently we state in the FAQ section that the oldest Unity version supported is 2020.3, but on the Unity website it states that 2020.3 LTS stream has reached End-of-Support and we also have a limitation: the IL2CPP builds cannot be created using the Managed stripping level set to Minimal, as the Minimal level does not exist in Unity 2020 - as a consequence the users cannot connect their IL2CPP builds with the AltTester driver if the stripping level is higher. There are 2 options, either:

• We update the oldest supported version to 2021.3 LTS
• We add the info about the limitation for the IL2CPP builds when using Unity 2020.3

Mocks

Additional info

The problem with the missing setting was reported by a user on Discord

[iulianatodoran]

@gomboskriszta15 I added the 2.1.3 label if that's ok