Bump mongoid from 8.1.3 to 9.0.2 in /backend

Bumps [mongoid](https://github.com/mongodb/mongoid) from 8.1.3 to 9.0.2.
- [Release notes](https://github.com/mongodb/mongoid/releases)
- [Changelog](https://github.com/mongodb/mongoid/blob/master/CHANGELOG.md)
- [Commits](mongodb/mongoid@v8.1.3...v9.0.2)

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

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

backend/Gemfile

@@ -14,7 +14,7 @@ gem "sprockets-rails"
 gem "active_model_serializers", "~> 0.9"
 
 # Use postgresql and mongo as the database for Active Record
-gem "mongoid", "8.1.3" # https://www.mongodb.com/docs/mongoid/current/reference/compatibility/#rails-compatibility
+gem "mongoid", "9.0.2" # https://www.mongodb.com/docs/mongoid/current/reference/compatibility/#rails-compatibility
 gem "pg"
 
 # Use Puma as the app server

backend/Gemfile.lock

@@ -99,10 +99,10 @@ GEM
       erubi (~> 1.4)
       parser (>= 2.4)
       smart_properties
-    bigdecimal (3.1.7)
+    bigdecimal (3.1.8)
     brakeman (6.1.2)
       racc
-    bson (4.15.0)
+    bson (5.0.1)
     bugsnag (6.27.1)
       concurrent-ruby (~> 1.0)
     builder (3.3.0)
@@ -126,7 +126,7 @@ GEM
     coercible (1.0.0)
       descendants_tracker (~> 0.0.1)
     colored (1.2)
-    concurrent-ruby (1.3.3)
+    concurrent-ruby (1.3.4)
     connection_pool (2.4.1)
     countries (4.0.1)
       i18n_data (~> 0.13.0)
@@ -218,7 +218,7 @@ GEM
     hashdiff (1.1.0)
     hashie (3.5.7)
     httpclient (2.8.3)
-    i18n (1.14.5)
+    i18n (1.14.6)
       concurrent-ruby (~> 1.0)
     i18n_data (0.13.0)
     io-console (0.7.2)
@@ -254,14 +254,13 @@ GEM
     method_source (1.1.0)
     mini_mime (1.1.5)
     mini_portile2 (2.8.7)
-    minitest (5.24.1)
-    mongo (2.20.1)
+    minitest (5.25.1)
+    mongo (2.21.0)
       bson (>= 4.14.1, < 6.0.0)
-    mongoid (8.1.3)
-      activemodel (>= 5.1, < 7.2, != 7.0.0)
+    mongoid (9.0.2)
+      activemodel (>= 5.1, < 7.3, != 7.0.0)
       concurrent-ruby (>= 1.0.5, < 2.0)
       mongo (>= 2.18.0, < 3.0.0)
-      ruby2_keywords (~> 0.0.5)
     mongoid-compatibility (0.6.0)
       activesupport
       mongoid (>= 2.0)
@@ -525,7 +524,7 @@ DEPENDENCIES
   kaminari-actionview
   kaminari-mongoid
   letter_opener
-  mongoid (= 8.1.3)
+  mongoid (= 9.0.2)
   mongoid-rspec
   nearest_time_zone
   omniauth (~> 1.8)