Use new `process.on('SIGTERM')` to flush on Vercel

[mydea]

Description

See https://vercel.com/changelog/vercel-functions-now-support-graceful-shutdown, Vercel has added a new way to handle graceful shutdown. We should use it to flush events in Sentry effectively:

process.on('SIGTERM', () => {
  Sentry.flush();
});

Things to figure out:

• Where should we call this? I guess this is not Next.js specific. So I would propose to add this in both Node as well as vercel-edge SDKs guarded by an env-var check for process.env.VERCEL?
• We should test that this actually works (in both edge and node)
• We should remove eventually remaining code to handle this in a more hacky way, using waitUntil or similar for vercel

[linear]

JS-916 Use new `process.on('SIGTERM')` to flush on Vercel

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #17583, which was included in the 10.33.0 release.