Bump notifications-ruby-client from 5.4.0 to 6.0.0 (#723)

Bumps [notifications-ruby-client](https://github.com/alphagov/notifications-ruby-client) from 5.4.0 to 6.0.0.
- [Changelog](https://github.com/alphagov/notifications-ruby-client/blob/main/CHANGELOG.md)
- [Commits](alphagov/notifications-ruby-client@5.4.0...6.0.0)

---
updated-dependencies:
- dependency-name: notifications-ruby-client
  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>

Gemfile.lock

@@ -260,7 +260,7 @@ GEM
       railties (>= 4.2.0)
       thor (>= 0.14, < 2.0)
     json (2.7.1)
-    jwt (2.7.0)
+    jwt (2.7.1)
     language_server-protocol (3.17.0.3)
     loofah (2.22.0)
       crass (~> 1.0.2)
@@ -296,7 +296,7 @@ GEM
       racc (~> 1.4)
     nokogiri (1.16.0-x86_64-linux)
       racc (~> 1.4)
-    notifications-ruby-client (5.4.0)
+    notifications-ruby-client (6.0.0)
       jwt (>= 1.5, < 3)
     octokit (4.25.1)
       faraday (>= 1, < 3)