Update `2.x` references on `3.x` branch

[Kocal]

After creating the 3.x branch, we must update many places where the 2.x branch is mentioned:
https://github.com/search?q=repo%3Asymfony%2Fux%202.x&type=code

[smnandre]

Only when we will publish it, right ? (just to be sure)

[Kocal]

After creating the 3.x branch

[smnandre]

I am not sure, we won't accept immediately PR on the 3.x I think ... won't we need just a bit of time to organize things ?

[Kocal]

I mean, we have to create the 3.x branch first on GitHub in order to do these changes, 5min or 5 days later, whatever, I'm not telling we should immediately do these changes when the 3.x branch is created.

[smnandre]

That was my question :) Sorry if i was not clear 💐