blocksuite-react vercel preview removal

[himself65]

As in #818, we want to merge the next code into the playground package. However, the next.js build is necessary because we must ensure stability in SSR mode in case of a building error in the AFFiNE repo.

So the compromise is we build the next in GitHub CI instead of the vercel build. See 237eccf. This will cause the next preview to be temporarily unavailable until we really move the code from next to the playground.

/cc @doodlewind

[doodlewind]

If we are still supporting Next, it looks fine with GitHub CI and Vercel build config both enabled, which is our current state.