Implement upgrade-time logic to strip any unknown fields from persisted cluster parameters that may have been written by older MongoDB versions while their IDL parsers were non-strict.

This is part of the broader effort to safely enable strict parsing for all cluster parameters in order to avoid bugs like SERVER-117009 and SERVER-116411. ** This broader effort involves:

• Rejecting unknown fields at the setClusterParameter entry point (addressed by SERVER-117009)
• Stripping unknown fields during upgrade (this Jira).
• Eventually enabling strict parsing on all cluster parameters and removing temporary compatibility logic once v9.0 branches out.

The work in this ticket should:

• Detect persisted cluster parameter documents containing fields that are no longer recognized under strict parsing.
• Remove those unknown fields as part of the upgrade process, so that post-upgrade binaries only see validated/known fields.

Additional context

After initially enabling strict parsing for all cluster parameters in a previous change (83d9829), we discovered that clusters could crash after upgrade if an unknown field (set while running an earlier, non-strict binary) was still present in the persisted cluster parameter state.
That change was therefore reverted.
To safely re-enable strict parsing we must ensure that no legacy, unknown fields survive across the upgrade boundary.