Bug 1867974 - Bump enigmail version again. r=me

src/releases/seamonkey2.53.18/index.en.html

@@ -226,8 +226,8 @@ note the following warnings and advice regarding extensions:</p>
   </li>
   <li>For <b>Enigmail</b>, please refer to the
     <a href="https://enigmail.net/index.php/en/home">Enigmail</a> page.
-    <br>The latest supported version is 3.1.7
-    <a href="https://enigmail.net/download/release/3.1/enigmail-3.1.7-il.xpi">
+    <br>The latest supported version at the time of the release is currently 3.1.8
+    <a href="https://enigmail.net/download/release/3.1/enigmail-3.1.8-esi.xpi">
     Enigmail</a>.
   </li>
   <li>Adblock Plus 2.9.1 may cause huge memory and CPU consumption and is not