Mark `EthKeyringController` as in Maintenance

[mikesposito]

After finishing #3766, the EthKeyringController repo should be marked maintenance until it is removed from clients, leaving a notice in the Readme linking to the core KeyringController.

This work is part of this plan: #2043

[desi]

We shouldn't archive the repo until it is no longer in production. We can add a notice that it is in maintenance only until ETHKeyringController is no longer in the dependency tree.

[desi]

Hey team! Please add your planning poker estimate with Zenhub @Gudahtt @kanthesha @MajorLift @mcmire @mikesposito