Change Tor Browser's recommended bridge type to obfs4

After legacy/trac#18071 (moved) is merged, I think we should change the recommended bridge type in Tor Browser to obfs4, given that we now have several high capacity obfs4 bridges and obfs4 is more likely to work in more regions than obfs3. Additionally, if we make it the default type, we're signalling to users that it's what they should be using. Also, obfs4 is the default PT type distributed by BridgeDB.