Update the reference for `atala` term in the identus-mediator repository

[shaileshp0110]

There still reference for atala prism in the read me

[FabioPinheiro]

@mineme0110 is there anything to update before releasing?

[yshyn-iohk]

There are a couple of references to the atala term in the README.md and application.conf files

It's also related to the PURI of the DIDComm protocols, which we plan to fix in the next milestone release.
The ticket is moved to the backlog of the Roadmap project.

CC: @FabioPinheiro, @mineme0110