Change chat room links to Zulip and update languages (#447)

Updated chat links to reflect new platforms and languages as we migrate
from Gitter to Zulip.

---------

Co-authored-by: askdkc <7894265+askdkc@users.noreply.github.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: askdkc

community/index.md

@@ -32,11 +32,7 @@ Here is a list of PGroonga related GitHub issues:
 
 ### Chat room {#chat}
 
-Here are chat rooms for PGroonga:
-
-  * For English speaker: [groonga/en chat room on Gitter](https://gitter.im/groonga/en)
-
-  * For Japanese speaker: [groonga/ja chat room on Gitter](https://gitter.im/groonga/ja)
+[Zulip](https://groonga.zulipchat.com/) is the chart room for PGroonga.
 
 ## How to get the latest information