UX: Various improvements to welcome topic CTA (#21010)

- Update welcome topic copy
- Edit the welcome topic automatically when the title or description changes
- Remove “Create your Welcome Topic” banner/CTA
- Add "edit welcome topic" user tip

app/models/site.rb

@@ -216,40 +216,4 @@ class Site
     # the cache is validated based on the sequence
     MessageBus.publish(SITE_JSON_CHANNEL, "")
   end
-
-  def self.welcome_topic_banner_cache_key(user_id)
-    "show_welcome_topic_banner:#{user_id}"
-  end
-
-  def self.welcome_topic_exists_and_is_not_edited?
-    Post
-      .joins(:topic)
-      .where(
-        "topics.id = :topic_id AND topics.deleted_at IS NULL AND posts.post_number = 1 AND posts.version = 1 AND posts.created_at > :created_at",
-        topic_id: SiteSetting.welcome_topic_id,
-        created_at: 1.month.ago,
-      )
-      .exists?
-  end
-
-  def self.clear_show_welcome_topic_cache
-    Discourse
-      .cache
-      .keys("show_welcome_topic_banner:*")
-      .each { |key| Discourse.cache.redis.del(key) }
-  end
-
-  def self.show_welcome_topic_banner?(guardian)
-    return false if !guardian.is_admin?
-    return false if guardian.user.id != User.first_login_admin_id
-    user_id = guardian.user.id
-
-    show_welcome_topic_banner = Discourse.cache.read(welcome_topic_banner_cache_key(user_id))
-    return show_welcome_topic_banner unless show_welcome_topic_banner.nil?
-
-    show_welcome_topic_banner = welcome_topic_exists_and_is_not_edited?
-
-    Discourse.cache.write(welcome_topic_banner_cache_key(user_id), show_welcome_topic_banner)
-    show_welcome_topic_banner
-  end
 end