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

Author: danielgustafsson

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>.

src/include/tsearch/dicts/spell.h

@@ -37,7 +37,7 @@ typedef struct
 
 /*
  * Names of FF_ are correlated with Hunspell options in affix file
- * http://hunspell.sourceforge.net/
+ * https://hunspell.github.io/
  */
 #define FF_COMPOUNDONLY		0x01
 #define FF_COMPOUNDBEGIN	0x02