[lofoneh]

Fixes #130.

After an ICU upgrade on a libc database, every built-in *-x-icu collation
still records the old version, so collation_version_mismatch warned in every
database and listed all 871 of them. Nothing uses those collations, so nothing
can be out of order.

Now a changed collation only counts if a column, index, partition key or
something in pg_depend uses it. If nothing does, it's one info line that
doesn't affect the exit code. A used one still warns and the database default
is still critical.

One catch: Postgres doesn't record pg_depend rows for built-in collations
like PG16's unicode, so the query also checks pg_attribute and
pg_partitioned_table directly. The new integration test covers that case.

This adds an unreferenced field to the JSON, so the schema goes to 1.6.0.

I tested on PG 18.6 with all 871 ICU collations forced stale. Before: a warn
with 871 evidence lines. After: one info line. The full suite, doc-verify,
lint and gate.sh pass.