Fix warnings related to Query.get() in backend

[larsmoan]

We have a bunch of these warnings from the testing of our backend:
"
/home/runner/work/P03-Brewbucks/P03-Brewbucks/backend/brewbucks/views/routes.py:101: LegacyAPIWarning: The Query.get() method is considered legacy as of the 1.x series of SQLAlchemy and becomes a legacy construct in 2.0. The method is now available as Session.get() (deprecated since: 2.0) (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
user = Users.query.get(user_id)
"

Query.get() is legacy, we should use Session.get()

Not critical, but shouldn't be too much of an issue altering it.