[doc] We use GitHub for discussion not the mailing list

Fixes https://github.com/harfbuzz/harfbuzz/issues/3160

docs/usermanual-install-harfbuzz.xml

@@ -24,8 +24,7 @@
       overview of the changes. The project <ulink url="https://github.com/harfbuzz/harfbuzz/issues">tracks bug
       reports and other issues</ulink> on GitHub. Discussion and
       questions are welcome on the <ulink
-      url="http://freedesktop.org/mailman/listinfo/harfbuzz/">HarfBuzz
-      mailing list</ulink>.
+      url="https://github.com/harfbuzz/harfbuzz/discussions">GitHub</ulink> as well.
     </para>
     <para>
       The API included in the <filename