Implement updateUser command

[b1ron]

What should be done?

With passwords being stored, we should implement the basic updateUser command.

Also, in this issue, we should update all integration tests to use createUser, dropUser, dropAllUsersFromDatabase, usersInfo commands instead of using find/insert/delete/etc on the admin.system.users collection.

Where?

https://github.com/FerretDB/FerretDB/tree/main/internal/handler

Definition of Done

• handler updated;
• integration/compatibility tests added/updated;
• spot refactorings done.