Only use MinimalAggregatorV3Interface

[Jean-Grimal]

Fixes #94

[QGarchery]

So we decided to use 2 folders so that it avoids the risk of modifying the source code when working on a new contract. Here this is making it possible again, because one can be tempted to modify IStEth for a new oracle that needs other data from stEth.

Does it make sense here to share the interfaces ? Not sure what we gain, but at the same time adding function signatures to the interface should not be a problem at all

[MerlinEgalite]

Good point.

But I remember we decided to use the minimal interface because some functions were useless in the standard interface.

Right now, when integrators fork the repo to build a new oracle they use the standard interface because it's in morpho-chainlink folder and, thus, implement useless functions (eg Angle, fxusd, etc.). I would recommend to at least change the standard interface to the minimal in the folder mentioned earlier.

[MathisGD]

@Jean-Grimal can you make the change please