Extract "migrations" config name to a proper constant

Jellyfin.Server/Migrations/MigrationRunner.cs

@@ -27,7 +27,7 @@ namespace Jellyfin.Server.Migrations
         public static void Run(CoreAppHost host, ILoggerFactory loggerFactory)
         {
             var logger = loggerFactory.CreateLogger<MigrationRunner>();
-            var migrationOptions = ((IConfigurationManager)host.ServerConfigurationManager).GetConfiguration<MigrationOptions>("migrations");
+            var migrationOptions = ((IConfigurationManager)host.ServerConfigurationManager).GetConfiguration<MigrationOptions>(MigrationsListStore.StoreKey);
             var applied = migrationOptions.Applied.ToList();
 
             for (var i = 0; i < Migrations.Length; i++)
@@ -56,7 +56,7 @@ namespace Jellyfin.Server.Migrations
             {
                 logger.LogInformation("Some migrations were run, saving the state");
                 migrationOptions.Applied = applied.ToArray();
-                host.ServerConfigurationManager.SaveConfiguration("migrations", migrationOptions);
+                host.ServerConfigurationManager.SaveConfiguration(MigrationsListStore.StoreKey, migrationOptions);
             }
         }
     }

Jellyfin.Server/Migrations/MigrationsListStore.cs

@@ -7,13 +7,18 @@ namespace Jellyfin.Server.Migrations
     /// </summary>
     public class MigrationsListStore : ConfigurationStore
     {
+        /// <summary>
+        /// The name of the configuration in the storage.
+        /// </summary>
+        public static readonly string StoreKey = "migrations";
+
         /// <summary>
         /// Initializes a new instance of the <see cref="MigrationsListStore"/> class.
         /// </summary>
         public MigrationsListStore()
         {
             ConfigurationType = typeof(MigrationOptions);
-            Key = "migrations";
+            Key = StoreKey;
         }
     }
 }