Update dbinit-base to rel/v16.1/rc0

[smklein]

Generated with claude, using the following (and only) prompt:

Read schema/crdb/README.adoc, in particular the section on updating dbinit-base.sql.
Please update the base to git tag "rel/v16.1/rc0".
Then, ensure "cargo check --all-targets" compiles clean, and run/fix the schema and data migration tests.

This is sorta a cheap test for "are the docs and checks clear enough".

Claude ran the following:

• cargo xtask schema generate-base rel/v16.1/rc0
• cargo xtask schema old-migrations
• Removed all the old migrations, and updated necessary constants, from nexus/db-model
• cargo xtask schema old-migrations --remove
• cargo check --all-targets (compiled cleanly)
• cargo nextest run -p omicron-nexus 'integration_tests::schema' (it observed the data migration test failing, as expected)
• Removed old data migration modules
• cargo nextest run -p omicron-nexus 'integration_tests::schema' (now passing)

This pretty much exactly matches what I'd expect/want a human do to, for this process.