Change HTML sanitization to remove unusable and unused `embed` tag (#34021)

lib/sanitize_ext/sanitize_config.rb

@@ -94,19 +94,17 @@ class Sanitize
       ]
     )
 
-    MASTODON_OEMBED ||= freeze_config(
-      elements: %w(audio embed iframe source video),
+    MASTODON_OEMBED = freeze_config(
+      elements: %w(audio iframe source video),
 
       attributes: {
         'audio'  => %w(controls),
-        'embed'  => %w(height src type width),
         'iframe' => %w(allowfullscreen frameborder height scrolling src width),
         'source' => %w(src type),
         'video'  => %w(controls height loop width),
       },
 
       protocols: {
-        'embed'  => { 'src' => HTTP_PROTOCOLS },
         'iframe' => { 'src' => HTTP_PROTOCOLS },
         'source' => { 'src' => HTTP_PROTOCOLS },
       },