Advance base schema version to rel/v18.1/rc1

[jgallagher]

Thanks @smklein for the claude-one-shot instructions:

There are docs for this in https://github.com/oxidecomputer/omicron/tree/main/schema/crdb#21-updating-the-baseline-after-a-release

I've had success one-shotting this (it's mechanical) using claude via:

Read schema/crdb/README.adoc, in particular the section on updating dbinit-base.sql.
Please update the base to git tag "PICK_YOUR_GIT_TAG".
Then, ensure "cargo check --all-targets" compiles clean, and run/fix the schema and data migration tests.