chore(deps): bump ash_graphql from 0.26.6 to 0.28.1 in /backend-elixir

Bumps [ash_graphql](https://github.com/ash-project/ash_graphql) from 0.26.6 to 0.28.1.
- [Changelog](https://github.com/ash-project/ash_graphql/blob/v0.28.1/CHANGELOG.md)
- [Commits](ash-project/ash_graphql@v0.26.6...v0.28.1)

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

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

backend-elixir/mix.exs

@@ -33,7 +33,7 @@ defmodule StudyHall.MixProject do
   defp deps do
     [
       {:ash_authentication, "~> 3.11"},
-      {:ash_graphql, "~> 0.26.0"},
+      {:ash_graphql, "~> 0.28.1"},
       {:ash_phoenix, "~> 1.2"},
       {:ash_postgres, "~> 1.3"},
       {:ash, "~> 2.14"},