Bug 1717984: update/replace dead links in the 2.53.8 release notes. r=IanN

- Two links are updated to point to the current location of the same
  content at mozilla.org;

- The link for comm-central checkout instructions is replaced with a
  link to the "Central Branch" section of the build instructions
  introduced in bug 1620789.

src/releases/seamonkey2.53.8/index.en.html

@@ -578,7 +578,8 @@ Please read this before reporting any new bugs.
     <dd>
     We need help from developers and the testing community to provide as much
     feedback as possible to make SeaMonkey even better. Please read these notes
-    and the <a href="http://developer.mozilla.org/en/Bug_writing_guidelines">bug
+    and the <a
+    href="https://developer.mozilla.org/en-US/docs/Mozilla/QA/Bug_writing_guidelines">bug
     filing instructions</a> before reporting any bugs to
     <a href="https://bugzilla.mozilla.org/">Bugzilla</a>.
     </dd>
@@ -612,8 +613,8 @@ Please read this before reporting any new bugs.
     <dd>
     A tarball of the [% rnote.name %] source code is available at the
     <a href="../[% rnote.version %]#source">download page</a>.
-    The latest development code can be obtained from the
-    <a href="http://developer.mozilla.org/en/Comm-central_source_code_(Mercurial)">source
+    The latest development code can be obtained from the <a
+    href="https://www.seamonkey-project.org/dev/code-development#central">source
     repository</a>.
     SeaMonkey-specific source is in the "suite" directory. Please follow the
     <a href="http://developer.mozilla.org/en/docs/Build_Documentation">build
@@ -641,7 +642,8 @@ The following resources contain useful information about [% rnote.name %]:
 <!--  <li><a href="https://wiki.mozilla.org/SeaMonkey/Features/[% rnote.version %]">&quot;SeaMonkey/Features/[% rnote.version %]&quot;
     wiki page</a>
   </li> --->
-  <li><a href="http://developer.mozilla.org/En/Extensions_support_in_SeaMonkey_2">For
+  <li><a
+    href="https://developer.mozilla.org/en-US/docs/Archive/Add-ons/SeaMonkey_2">For
     extension developers</a>
   </li>
 </ul>