Stabilize the new output formats

[ntBre]

The new output formats built on the new diagnostic model are currently gated behind preview. We should stabilize them in the next minor release. See #19133 as one example.

[ntBre]

This is still only used by the JSON output format, but it was added in the middle of the last release cycle. I think we can defer this until 0.14 both to give JSON users more time to report issues and to make sure we don't need the preview diagnostic configuration option for anything else. Currently, stabilizing the small JSON preview changes would leave DisplayDiagnosticConfig::preview completely unused.