UX: Remove welcome topic admin tip and tweak copy (#21593)

The welcome topic user tip was for admins only, but in general, user
tips should be used for guiding new users through the features that
Discourse offers. For this reason, we decided to remove the user tip.

This commit also includes a few more copy tweaks to the welcome topic.

spec/requests/api/schemas/json/site_response.json

@@ -241,9 +241,6 @@
         },
         "suggested_topics": {
           "type": "integer"
-        },
-        "welcome_topic": {
-          "type": "integer"
         }
       },
       "required": [