The `deprecate_column` helper would change its behavior based on the current `Discourse::VERSION`. This means that 'finalizing' a stable release introduces a previously untested behavior change.
Much better to keep it as a deprecation until manual action is taken to introduce the breaking change.
Currently when we decide we're going to drop a column in the future we just mark it with a TODO comment and add it to ignored_columns. This makes it instantly unavailable, and we mostly forget about the TODO in the end. 😬
This change adds a HasDeprecatedColumns concern which offers a little bit more flexibility. We can still simulate the old behaviour by setting drop_from to the current version, but we can also set it to a future version, causing it to raise a deprecation warning until then if used.
