Bump pg from 1.2.3 to 1.5.6 in /backend (#730)

Bumps [pg](https://github.com/ged/ruby-pg) from 1.2.3 to 1.5.6.
- [Changelog](https://github.com/ged/ruby-pg/blob/master/History.md)
- [Commits](ged/ruby-pg@v1.2.3...v1.5.6)

---
updated-dependencies:
- dependency-name: pg
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

backend/Gemfile

@@ -14,7 +14,7 @@ gem "active_model_serializers", "0.9.8"
 
 # Use postgresql and mongo as the database for Active Record
 gem "mongoid", "7.3.3"
-gem "pg", "1.2.3"
+gem "pg", "1.5.6"
 
 # Use Puma as the app server
 gem "puma", "5.6.8"

backend/Gemfile.lock

@@ -296,9 +296,8 @@ GEM
     parser (3.2.2.4)
       ast (~> 2.4.1)
       racc
-    pg (1.2.3)
-    pg (1.2.3-x64-mingw32)
-    pg (1.2.3-x86-mingw32)
+    pg (1.5.6)
+    pg (1.5.6-x86-mingw32)
     pry (0.14.2)
       coderay (~> 1.1)
       method_source (~> 1.0)
@@ -522,7 +521,7 @@ DEPENDENCIES
   nearest_time_zone
   omniauth (= 1.8.1)
   omniauth-facebook (= 3.0.0)
-  pg (= 1.2.3)
+  pg (= 1.5.6)
   pry-byebug
   pry-doc
   pry-rails