[release] Version 6.0.0

Migrate to firebase v9 modular sdk (#243)

* [WIP] Update to modular SDK

* Lint

* Version 9.0.0-beta.1

* Package lock

* test

* replace uglify with terser

* use stable version of firebase v9

* bump ci to node 10

* Reference --> DatabaseReference in tests

* unsubscribe from listeners instead of calling off()

* callback --> unsubscribe

* add migration guide

* update reference docs

* update example fish1

* update remaining examples

* update common/package-lock

* update package-lock.json files

* Update migration.md

* Update packages/geofire/src/GeoQuery.ts

* add changelog entry

Co-authored-by: Sam <[email protected]>

[release] Version 5.2.0

[release] Version 5.1.0

[firebase-release] Updated GeoFire to 5.0.1

[firebase-release] Updated GeoFire to 5.0.0

[firebase-release] Updated GeoFire to 4.1.2

[firebase-release] Updated GeoFire to 4.1.1

[firebase-release] Updated GeoFire to 4.1.0

[firebase-release] Updated GeoFire to 4.0.0