Update Crystal documentation (1.21.0)

Author: simon04

lib/docs/filters/crystal/clean_html.rb

@@ -2,6 +2,7 @@ module Docs
   class Crystal
     class CleanHtmlFilter < Filter
       def call
+        css('a:contains("Edit this page")').remove
         current_url.path.start_with?('/reference/') ? book : api
         doc
       end

lib/docs/scrapers/crystal.rb

@@ -2,7 +2,7 @@ module Docs
   class Crystal < UrlScraper
     include MultipleBaseUrls
     self.type = 'crystal'
-    self.release = '1.20.0'
+    self.release = '1.21.0'
     self.base_urls = [
       "https://crystal-lang.org/api/#{release}/",
       "https://crystal-lang.org/reference/#{release[0..2]}/",