Replace Twitter link by Fosstodon one (#973)

Co-authored-by: Even Rouault <[email protected]>

Author: github-actions[bot]

en/index.txt

@@ -26,7 +26,7 @@ lists, etc.
    :alt: Digital Object Identifier (DOI) for MapServer
 
 .. tip::
-    Follow MapServer on `Twitter <https://twitter.com/mapserver_osgeo/>`__
+    Follow MapServer on `Fosstodon <https://fosstodon.org/@[email protected]>`__
 
 :ref:`More about MapServer <about>`