Updating eth2-testnets url

[fredrik0x]

https://github.com/eth2-clients was renamed to https://github.com/eth-clients

[cla-bot-2021]

User @fredriksvantes, please sign the CLA here.

[kirushik]

Hey, @fredriksvantes, did you actually mean to point the URL to https://github.com/eth-clients/eth2-networks?

The very fact that we point to the obsolete repo name just highlights the issue of us not maintaining Shasper since approximately 2019.
@sorpaas maybe we should mark this repository as archived instead?

[fredrik0x]

Yeah,

Hey, @fredriksvantes, did you actually mean to point the URL to https://github.com/eth-clients/eth2-networks?

The very fact that we point to the obsolete repo name just highlights the issue of us not maintaining Shasper since approximately 2019. @sorpaas maybe we should mark this repository as archived instead?

Ah yes, that was indeed the intention! I updated the PR in case you don't archive it.