Use span-based string.Concat to avoid unnecessary allocation

Calling Substring produces a copy of the extracted substring. By using AsSpan instead of Substring and calling the overload of string.Concat that accepts spans, you can eliminate the unnecessary string allocation.

(cherry picked from commit e8aff90582fb50b2d48dea3a4c2139c2745f1554)

Fixes #2262

src/NzbDrone.Common/Http/HttpUri.cs

@@ -170,7 +170,7 @@ namespace NzbDrone.Common.Http
 
             if (baseSlashIndex >= 0)
             {
-                return basePath.Substring(0, baseSlashIndex) + "/" + relativePath;
+                return $"{basePath.AsSpan(0, baseSlashIndex)}/{relativePath}";
             }
 
             return relativePath;