Improve merge UX by removing `--merge` and adding preparedness indicators

[michaelsproul]

Description

Here's a plan to improve merge UX:

• deprecate the --merge flag, by enabling everything it does currently by default
• clean up the logs for merge-related functionality so that:
  • if a Bellatrix epoch is set, we log loudly about the absence of EE endpoints, suggested fee recipients, etc
  • if no Bellatrix epoch is set, we do not log loudly about the absence of merge-related config (this might involve disabling some services that were previously enabled by the --merge flag)
• add an HTTP endpoint and/or routine log message to reassure users that their node is "merge ready", which checks the presence of all necessary flags

[paulhauner]

deprecate the --merge flag, by enabling everything it does currently by default

This was completed in #3214 🎉

[ronaldjmaas]

Also relating to #3315 : as Lighthouse does not support multiple execution endpoints anymore, please update the description for the following arguments accordingly:
--execution-endpoints
--jwt-secrets

and post merge:
--eth1-endpoints

[michaelsproul]

@ronaldjmaas The --help for those flags has already been updated, in fact they're absent from --help as they are now just aliases to the new flags. The release notes for the next release will note this backwards-incompatible change.

        --execution-endpoint <EXECUTION-ENDPOINT>
            Server endpoint for an execution layer jwt authenticated HTTP JSON-RPC connection. Uses the same endpoint to
            populate the deposit cache. Also enables the --merge flag.If not provided, uses the default value of
            http://127.0.0.1:8551

        --execution-jwt <EXECUTION-JWT>
            File path which contains the hex-encoded JWT secret for the execution endpoint provided in the --execution-
            endpoint flag.

I think --eth1-endpoints needs its description updated soon too.

[pawanjay176]

add an HTTP endpoint and/or routine log message to reassure users that their node is "merge ready", which checks the presence of all necessary flags

This was completed in #3349 🎉

[michaelsproul]

I think the original purpose of this issue has been accomplished! We can continue tweaking merge UX over the next few weeks