add variable for exit-only consensus

[isstuev]

Description and Related Issue(s)

resolves #3161

Proposed Changes

added NEXT_PUBLIC_BEACON_CHAIN_WITHDRAWALS_ONLY variable to disable deposits for beacon chain

Breaking or Incompatible Changes

Additional Information

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request

[tom2drum]

I think we also need to update the condition in the deposits guard function -

frontend/nextjs/getServerSideProps/guards.ts

Line 219 in e540c01

export const deposits: Guard = (chainConfig: typeof config) => async() => {