Refactoring: Add extension method for retrieval of language ISO codes if that's all we need (#20324)

* Retrieve only ISO codes from the database rather than full language objects if that's all we need.

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Removed repository updates and migrated the new service method to an extension method.

* Fixed issue after merge.

* Removed left-over using

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Kenn Jacobsen <kja@umbraco.dk>

src/Umbraco.Core/Extensions/LanguageServiceExtensions.cs

@@ -0,0 +1,16 @@
+using Umbraco.Cms.Core.Services;
+
+namespace Umbraco.Cms.Core.Extensions;
+
+/// <summary>
+/// Provides extension methods for <see cref="ILanguageService"/>.
+/// </summary>
+public static class LanguageServiceExtensions
+{
+    /// <summary>
+    /// Retrieves all ISO codes of all languages.
+    /// </summary>
+    /// <param name="service">The <see cref="ILanguageService"/>.</param>
+    /// <returns>A collection of language ISO codes.</returns>
+    public static async Task<IEnumerable<string>> GetAllIsoCodesAsync(this ILanguageService service) => (await service.GetAllAsync()).Select(x => x.IsoCode);
+}