Added Contains compatible with old .NET version

Author: JaBistDuNarrisch

src/Migrator/Framework/Extensions/LinqExtensions.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace DotNetProjects.Migrator.Framework.Extensions;
+
+public static class LinqExtensions
+{
+    /// <summary>
+    /// Is equal to the Contains method in .NET 9. Please remove it after .NET upgrade.
+    /// </summary>
+    /// <param name="source"></param>
+    /// <param name="toBeChecked"></param>
+    /// <param name="stringComparison"></param>
+    /// <returns></returns>
+    public static bool Contains(this string source, string toBeChecked, StringComparison stringComparison)
+    {
+        return source?.IndexOf(toBeChecked, stringComparison) >= 0;
+    }
+}