Bump flipper from 0.26.2 to 0.28.0

Bumps [flipper](https://github.com/jnunemaker/flipper) from 0.26.2 to 0.28.0.
- [Release notes](https://github.com/jnunemaker/flipper/releases)
- [Changelog](https://github.com/jnunemaker/flipper/blob/main/Changelog.md)
- [Commits](flippercloud/flipper@v0.26.2...v0.28.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -182,14 +182,14 @@ GEM
     faraday_middleware (1.2.0)
       faraday (~> 1.0)
     ffi (1.15.5)
-    flipper (0.26.2)
+    flipper (0.28.0)
       concurrent-ruby (< 2)
-    flipper-active_record (0.26.2)
+    flipper-active_record (0.28.0)
       activerecord (>= 4.2, < 8)
-      flipper (~> 0.26.2)
-    flipper-ui (0.26.2)
+      flipper (~> 0.28.0)
+    flipper-ui (0.28.0)
       erubi (>= 1.0.0, < 2.0.0)
-      flipper (~> 0.26.2)
+      flipper (~> 0.28.0)
       rack (>= 1.4, < 3)
       rack-protection (>= 1.5.3, <= 4.0.0)
       sanitize (< 7)