Replace link to Hunspell with the current homepage

The Hunspell project moved from Sourceforge to Github sometime
in 2016, so update our links to match the new URL.  Backpatch
the doc changes to all supported versions.

Discussion: https://postgr.es/m/DC9A662A-360D-4125-A453-5A6CB9C6C4B4@yesql.se
Backpatch-through: v11

doc/src/sgml/textsearch.sgml

@@ -2902,7 +2902,7 @@ SELECT plainto_tsquery('supernova star');
     url="https://www.cs.hmc.edu/~geoff/ispell.html">Ispell</ulink>.
     Also, some more modern dictionary file formats are supported &mdash; <ulink
     url="https://en.wikipedia.org/wiki/MySpell">MySpell</ulink> (OO &lt; 2.0.1)
-    and <ulink url="https://sourceforge.net/projects/hunspell/">Hunspell</ulink>
+    and <ulink url="https://hunspell.github.io/">Hunspell</ulink>
     (OO &gt;= 2.0.2).  A large list of dictionaries is available on the <ulink
     url="https://wiki.openoffice.org/wiki/Dictionaries">OpenOffice
     Wiki</ulink>.