Remove gcp: us-central1 c2 (new cronut) from new tenant options for data planes

[jwhartley]

We'd like to migrate off this data plane in the fullness of time given there are some networks limitations that we'd like to avoid, let's stop giving new tenants access to it.

Slack for context: https://estuaryworkspace.slack.com/archives/C03Q9A3GFEE/p1763652377827709?thread_ts=1763464381.379229&cid=C03Q9A3GFEE