install: Remove references to "Zulip Voyager".

"Zulip Voyager" was a name invented during the Hack Week to open
source Zulip for what a single-system Zulip server might be called, as
a Star Trek pun on the code it was based on, "Zulip Enterprise".

At the time, we just needed a name quickly, but it was never a good
name, just a placeholder.  This removes that placeholder name from
much of the codebase.  A bit more work will be required to transition
the `zulip::voyager` Puppet class, as that has some migration work
involved.

zerver/management/commands/checkconfig.py

@@ -5,7 +5,7 @@ from django.core.management.base import BaseCommand
 from zerver.lib.management import check_config
 
 class Command(BaseCommand):
-    help = """Checks your Zulip Voyager Django configuration for issues."""
+    help = """Checks /etc/zulip/settings.py for common configuration issues."""
 
     def handle(self, *args: Any, **options: Any) -> None:
         check_config()