Bump puma from 5.6.7 to 6.4.2 in /backend (#708)

Bumps [puma](https://github.com/puma/puma) from 5.6.7 to 6.4.2.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v5.6.7...v6.4.2)

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

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

backend/Gemfile

@@ -17,7 +17,7 @@ gem "mongoid", "7.3.3"
 gem "pg", "1.2.3"
 
 # Use Puma as the app server
-gem "puma", "5.6.8"
+gem "puma", "6.4.2"
 
 # Authentication libraries
 gem "cancancan", "~> 3.3.0"

backend/Gemfile.lock

@@ -313,9 +313,9 @@ GEM
     pry-rails (0.3.9)
       pry (>= 0.10.4)
     public_suffix (5.0.3)
-    puma (5.6.8)
+    puma (6.4.2)
       nio4r (~> 2.0)
-    puma (5.6.8-java)
+    puma (6.4.2-java)
       nio4r (~> 2.0)
     pusher (2.0.2)
       httpclient (~> 2.8)
@@ -523,7 +523,7 @@ DEPENDENCIES
   pry-byebug
   pry-doc
   pry-rails
-  puma (= 5.6.8)
+  puma (= 6.4.2)
   pusher
   rack-cors (= 2.0.1)
   rack-timeout