Update supported litmusctl version to 1.21.0 (#298)

Author: SarthakJain26

README.md

@@ -30,7 +30,7 @@ To check compatibility of litmusctl with Chaos Center
   <th>Lowest Chaos Center supported version</th>
   <th>Highest Chaos Center supported version</th>
  <tr>
-    <td>1.20.0</td>
+    <td>1.21.0</td>
     <td>3.0.0</td>
     <td>3.24.0</td>
  </tr>