Set version header to X-Application-Version (missing hyphen)

Not sure if this is intentional or really harms anything but to be consistent with other Arr apps like Sonarr and Radarr, this changes the version header to X-Application-Version.

src/Prowlarr.Http/Middleware/VersionMiddleware.cs

@@ -6,7 +6,7 @@ namespace Prowlarr.Http.Middleware
 {
     public class VersionMiddleware
     {
-        private const string VERSIONHEADER = "X-ApplicationVersion";
+        private const string VERSIONHEADER = "X-Application-Version";
 
         private readonly RequestDelegate _next;
         private readonly string _version;