cli: update parallel default for download:plugins script

[vince-fugnitto]

Feature Description:

The download:plugins script currently download and resolves extensions (packs + depedencies) in parallel, but this may cause issues with how open-vsx recently implemented rate limiting.

We should ultimately update both our downloading and @theia/vsx-registry to respect the limit, but a good first step is improve the situation when downloading during CI and locally by performing sequential downloads. This should fix the occasional 429 errors we experience.

[vince-fugnitto]

We introduced the sequential download by default #11860 and updated back to parallel with #11962 when we introduced client side rate limiting. I believe the issue can be closed, and downstream apps can always update the option.