OpenAPI, required on create schemas and 3.1 output

Follow up to #1078, which described everything php-crud-api can do except these
two. Both are larger than the phases that are done and neither of them is needed
for the document to be correct.

A faithful required on create schemas

A create schema has no required array. Getting one right means knowing which
columns have a default and which ones are identity columns. ReflectedColumn
carries neither and GenericReflection does not select them, so this means
touching the reflection query of all four drivers and the format of the
reflection cache. Until that is done any required list is a guess, which is
why there is none.

An openApiVersion setting for 3.1 output

The document says 3.0.0 and openApiBase can override that string, but
nothing changes in what is emitted. 3.1 spells a few things differently,
nullable becomes type: [x, "null"] and example becomes examples. Now
that the 3.0 document is complete, a setting can switch the dialect.