[v8] Bump `@sentry/webpack-plugin` in `@sentry/nextjs` & stop exposing options directly

[mydea]

We want to bump the webpack-plugin version to latest in Next.js, and also stop passing plugin options through directly so we can in the future update this easier without a breaking change.

[christianvuerings]

@mydea any updates on this? This is blocking us from using the latest @sentry/webpack-plugin v2 release

[mydea]

This will be done in the upcoming major release v8, as we can't update this in v7 because v2 of the webpack plugin has a different node support policy 😬 we'll keep you posted in here!

[christianvuerings]

@mydea Great, thank you - will monitor this issue