Make Synapse's url_preview_accept_language default to 'en-US, en'

Related to https://github.com/matrix-org/synapse/issues/11604

Getting an upstream fix is preferable. In any case, it's probably nice
to have this defined explicitly in our configuration. This way, people
can more easily discover that they can override the URL preview
language.

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -1149,8 +1149,7 @@ max_spider_size: 10M
 #   - fr;q=0.8
 #   - *;q=0.7
 #
-url_preview_accept_language:
-#   - en
+url_preview_accept_language: {{ matrix_url_preview_accept_language|to_json }}
 
 
 ## Captcha ##