Chain sunsetting

[Xeonus]

Define a plan on how to handle chain sunsetting moving forward and sync up with relevant teams (backend, front-end, ecosystem).

TODOs:

• Technical specification of "sunset chain"
• Stakeholder mgmt
• Plan on how to sunset zkEVM, Fraxtal and potentially Mode
• BIP

[Xeonus]

Working document here - needs further coordination with Backend team and ecosystem stakeholders