Rename useLsp setting to useLegacyAnalyzerProtocol and invert meaning #4097

A few issues have come up from users with LSP disabled (done because of earlier issues that are fixed, or following instructions shared as solutions to problems). To make it clearer that LSP is now the expected protocol and the original server protocol is legacy (as far as Dart-Code is concerned), we should rename this setting. This will also opt-in users that have previously opted out, but might not have intended to do so long-term.

(This is mainly a placeholder issue to ensure this change is noted in release notes).