schemachanger: Deprecate is_nullable in ColumnType element

[Xiang-Gu]

This issue is opened to track the deprecation process for is_nullable in ColumnType element.
The context here is that we changed the element modeling in V23_1 to handle NOT NULL constraint
of a column. We thus need to consider mixed version compatibility. This issue exists therefore to
track the deprecation process as we move into future major releases.

Jira issue: CRDB-23797

[Xiang-Gu]

The current plan, which might age out, is to do the following in V23_2:

• Delete is_nullable from ColumnType;
• Stop using ColumnType.elementCreationMetadata in scmutationexec