Rename "legacy juggler bridge" mentioned in docs

[dhmlau]

From our last maintainer's call, @mitsos1os suggested that the term "legacy juggler bridge" might give the wrong impression to users that the juggler won't be supported or not working well because of the "legacy" word.

@bschrammIBM (our technical writer previously) had made a similar comment.

Acceptance Criteria

• go through the occurrences of "legacy juggler bridge", rename it to simply "juggler bridge" if applicable. See search results: https://github.com/strongloop/loopback-next/search?p=1&q=%22legacy+juggler%22&unscoped_q=%22legacy+juggler%22
• go through codebase and make sure "legacy" is not used anymore.