Bump mongoid from 7.3.3 to 9.0.0 in /backend

Bumps [mongoid](https://github.com/mongodb/mongoid) from 7.3.3 to 9.0.0.
- [Release notes](https://github.com/mongodb/mongoid/releases)
- [Changelog](https://github.com/mongodb/mongoid/blob/master/CHANGELOG.md)
- [Commits](mongodb/mongoid@v7.3.3...v9.0.0)

---
updated-dependencies:
- dependency-name: mongoid
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

backend/Gemfile

@@ -13,7 +13,7 @@ gem "rake"
 gem "active_model_serializers", "0.9.8"
 
 # Use postgresql and mongo as the database for Active Record
-gem "mongoid", "7.3.3"
+gem "mongoid", "9.0.0"
 gem "pg"
 
 # Use Puma as the app server

backend/Gemfile.lock

@@ -86,7 +86,7 @@ GEM
     bigdecimal (3.1.7)
     brakeman (6.1.2)
       racc
-    bson (4.15.0)
+    bson (5.0.0)
     bugsnag (6.26.0)
       concurrent-ruby (~> 1.0)
     builder (3.2.4)
@@ -231,12 +231,12 @@ GEM
     mini_mime (1.1.5)
     mini_portile2 (2.8.5)
     minitest (5.22.3)
-    mongo (2.19.3)
-      bson (>= 4.14.1, < 5.0.0)
-    mongoid (7.3.3)
-      activemodel (>= 5.1, < 6.2)
-      mongo (>= 2.10.5, < 3.0.0)
-      ruby2_keywords (~> 0.0.5)
+    mongo (2.20.0)
+      bson (>= 4.14.1, < 6.0.0)
+    mongoid (9.0.0)
+      activemodel (>= 5.1, < 7.2, != 7.0.0)
+      concurrent-ruby (>= 1.0.5, < 2.0)
+      mongo (>= 2.18.0, < 3.0.0)
     mongoid-compatibility (0.6.0)
       activesupport
       mongoid (>= 2.0)
@@ -490,7 +490,7 @@ DEPENDENCIES
   kaminari-actionview
   kaminari-mongoid
   letter_opener
-  mongoid (= 7.3.3)
+  mongoid (= 9.0.0)
   mongoid-rspec
   nearest_time_zone
   omniauth (= 1.8.1)