Don't generate a migration for enum label order change

[roji]

Currently, when the label order of an enum change, we detect this as a label addition and generate a migraion:

ALTER TYPE foo ADD VALUE 'three' BEFORE 'one';

The recent improvements in EFCore.PG 9.0 mean that label order sometimes changes (see #3390), causing upgrade issues.

Flagged by @ValeriyGourov in #3390 (comment)