Document known supported version of Bitbucket Server for browser ext

Created by: sqs

Determine which versions of Bitbucket Server the browser extension is known to support in the section at https://docs.sourcegraph.com/integration/bitbucket_server#browser-extension:

Verified supported versions: Bitbucket Server N.NN. File an issue if you are seeing incompatibility on a different version.

N.NN can just be the version you initially test on. In the future we would like to make broader guarantees about supported versions, but for now the most important thing is to just document what's known now.