Fix Error related to Banned users with Expiration dates

[DrZoddiak]

Fixes #3788

The issue above was actually 2 separate issues.
First: BAN_DATE_FORMAT#format required a Date instead of an Instant, causing the first error.

Second: StoredUserListMixin was actually causing the ClassCastException because the List it redirects is actually a list of GameProfiles and not of Strings, which it later uses to remove an entry from a map wherein the Key is a GameProfile

[ImMorpheus]

@DrZoddiak Thank you for your contribution!

Changes look good but it would be best to create separate PRs since they are two different issues.

First: BAN_DATE_FORMAT#format required a Date instead of an Instant, causing the first error.

Note for reviewers: there's an overloaded method for Object but it does an instance check under the hood and throw an exception if the object is not a Date or an Number