Update bug-reporting links for gitlab

After the gitlab migration we need to update bug-reporting instructions.

• https://snowflake.torproject.org/#bugs
  https://gitweb.torproject.org/pluggable-transports/snowflake-webext.git/tree/static/index.html?h=webext-0.3.1&id=d2a9a8fd136ac6bbba393de3d51fb7ca85e17b8a#n75
• https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/wikis/home#reporting-bugs

I checked in snowflake.git and did not find anything that needs to be changed.