Replace OP Superchain feature with generic Multichain explorer

[tom2drum]

Description and Related Issue(s)

This PR replaces the OP Superchain–specific feature with a generic Multichain explorer feature. The feature still uses the same multichain aggregator and stats APIs and the same env vars; only naming and surface area are generalized so it can support any multichain cluster, not only the former OP Superchain interop cluster.

Proposed Changes

• Feature: Replaced opSuperchain with a new multichain feature in configs/app/features/ (same logic and env vars: NEXT_PUBLIC_MULTICHAIN_ENABLED, NEXT_PUBLIC_MULTICHAIN_CLUSTER).
• UI and code: Renamed components, types, mocks, and references from "OP Superchain" / "opSuperchain" to "Multichain" (e.g. ui/optimismSuperchain/ usage, feature key opSuperchain → multichain in nav, pages, guards, etc.).
• Env presets: Renamed env presets (e.g. .env.multichain_dev, .env.multichain_prod) and updated deploy/validator configs and preset-sync tooling to use the new naming.
• Docs/ENVs: No changes to docs/ENVS.md or env variable names; existing Multichain explorer env vars remain as documented.

Breaking or Incompatible Changes

None. The same environment variables (NEXT_PUBLIC_MULTICHAIN_ENABLED, NEXT_PUBLIC_MULTICHAIN_CLUSTER) are used. Deployments that relied on the opSuperchain feature flag/key need to switch to the multichain feature (or equivalent config) where that key is referenced.

Additional Information

N/A

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 a feature or functionality that is not privacy-compliant (e.g., tracking, analytics, third-party services), I have disabled it for private mode.
• 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

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