Use modular Firebase admin in v1.x

[kmjennison]

Is your feature request related to a problem? Please describe.

We should take advantage of modular Firebase admin:
https://firebase.google.com/docs/admin/migrate-node-v10#use-modules-instead-of-namespaces

Describe the solution you'd like and how you'd implement it
A clear and concise description of what you want to happen.

Currently, we do:

import * as admin from 'firebase-admin'

We should use:

import { initializeApp } from 'firebase-admin/app'

Is this a breaking change?
Would this require existing users to change how they're using next-firebase-auth?

[Edited] Yes, firebase-admin would need to be v10+.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

None

[louisgv]

Firebase just released v11: https://github.com/firebase/firebase-admin-node/releases/tag/v11.0.0

I will try to use it. If there's any issue, should I report them here or should I make a new one? @kmjennison

[kmjennison]

@louisgv Please open a separate issue if you have a problem with v11. Thanks!

[kmjennison]

Closed in #548.