Deprecate `iterable_contains_unrelated_type` and `list_remove_unrelated_type`

[srawlins]

in favor of collection_methods_unrelated_type

See my proposal to replace those two with the new rule in the lints package: dart-lang/core#787.

The best way to do this would be to ship a lints release with iterable_contains_unrelated_type and list_remove_unrelated_type replaced by collection_methods_unrelated_type, then deprecated them. This would result in less churn.

[pq]

Great point re: less churn. We'll want to be thoughtful about how we time updates to package:lints. (See also: #58947 which proposes we deprecate a few recommended lints.)

[pq]

@srawlins: I'm thinking this is a little close to the wire to pull off?