[v9] Remove `experimental_captureRequestError`

[lforst]

Description

https://github.com/getsentry/sentry-javascript/blob/f4c59000ca57b566c3ed05fe5a0b06acd75a2543/packages/nextjs/src/common/captureRequestError.ts#L49-L51

experimental_captureRequestError was deprecated and can be removed in v9.

[AbhiPrasad]

This was done with #14607 - closing this issue