Deprecate `users.gh_login` column

[carols10cents]

Depends on:

• Update the Find/Show User API to query users.username #13763
• Change new user signup flow to include a step for picking crates.io username #13768
• possibly other sub-issues of the tracking issue as well

After the dependent issues are complete, ensure all remaining uses of users.gh_login are removed from the Rust code. Once there are no more uses, deprecate/remove the column from the database.

[moskirathe]

can also consider refactoring to decouple / remove the model definition of OauthGithub from user.rs to a new oauth_github.rs model file while tackling this issue.