Bump countries from 4.0.1 to 6.0.0 in /backend

Bumps [countries](https://github.com/countries/countries) from 4.0.1 to 6.0.0.
- [Release notes](https://github.com/countries/countries/releases)
- [Changelog](https://github.com/countries/countries/blob/master/CHANGELOG.md)
- [Upgrade guide](https://github.com/countries/countries/blob/master/UPGRADE.md)
- [Commits](countries/countries@v4.0.1...v6.0.0)

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

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

backend/Gemfile.lock

@@ -112,9 +112,8 @@ GEM
     colored (1.2)
     concurrent-ruby (1.2.3)
     connection_pool (2.4.1)
-    countries (4.0.1)
-      i18n_data (~> 0.13.0)
-      sixarm_ruby_unaccent (~> 1.1)
+    countries (6.0.0)
+      unaccent (~> 0.3)
     crack (1.0.0)
       bigdecimal
       rexml
@@ -200,7 +199,6 @@ GEM
     httpclient (2.8.3)
     i18n (1.14.4)
       concurrent-ruby (~> 1.0)
-    i18n_data (0.13.0)
     json (2.7.1)
     jwt (2.3.0)
     kaminari-actionview (1.2.1)
@@ -409,7 +407,6 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    sixarm_ruby_unaccent (1.2.0)
     smart_properties (1.17.0)
     sprockets (4.2.1)
       concurrent-ruby (~> 1.0)
@@ -440,6 +437,7 @@ GEM
     tomorrowio_rb (0.0.3)
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)
+    unaccent (0.4.0)
     unicode-display_width (2.5.0)
     uniform_notifier (1.16.0)
     vcr (6.2.0)