Disable CHANGELOG.json by default

[ecraig12345]

Nothing in beachball reads the CHANGELOG.json file, and it's not that useful to users, while potentially using a lot of space in git history. (It's the only place where change entries with type none are recorded, but this probably has limited benefit; we could also add an option to include those in the markdown under "other changes" if desired.)

~~In the current version, we can add an option writeChangelogJson which defaults to true. In a major release, we can change the default of this option to false.~~

We have an option generateChangelog which currently defaults to true. #980 added 'md' and 'json' as possible values for this option, to allow writing only markdown (or only json if someone wants that for some reason).

In a major release, we can consider changing the default for generateChangelog to 'md'.