fix(nextjs): Use preprocessEvent hook to improve span data

[lforst]

We use the preprocessEvent hook to pass the most up to date data to users event processors and also not to trigger the logic that sets the transaction source to custom when the transaction name is updated in event processors.

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	22.73 KB	added	added
@sentry/browser - with treeshaking flags	21.53 KB	added	added
@sentry/browser (incl. Tracing)	35.12 KB	added	added
@sentry/browser (incl. Tracing, Replay)	71.83 KB	added	added
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	62.23 KB	added	added
@sentry/browser (incl. Tracing, Replay with Canvas)	76.19 KB	added	added
@sentry/browser (incl. Tracing, Replay, Feedback)	88.97 KB	added	added
@sentry/browser (incl. Tracing, Replay, Feedback, metrics)	90.8 KB	added	added
@sentry/browser (incl. metrics)	27 KB	added	added
@sentry/browser (incl. Feedback)	39.87 KB	added	added
@sentry/browser (incl. sendFeedback)	27.38 KB	added	added
@sentry/browser (incl. FeedbackAsync)	32.17 KB	added	added
@sentry/react	25.49 KB	added	added
@sentry/react (incl. Tracing)	38.09 KB	added	added
@sentry/vue	26.91 KB	added	added
@sentry/vue (incl. Tracing)	37.02 KB	added	added
@sentry/svelte	22.87 KB	added	added
CDN Bundle	24.11 KB	added	added
CDN Bundle (incl. Tracing)	36.96 KB	added	added
CDN Bundle (incl. Tracing, Replay)	71.62 KB	added	added
CDN Bundle (incl. Tracing, Replay, Feedback)	76.96 KB	added	added
CDN Bundle - uncompressed	70.7 KB	added	added
CDN Bundle (incl. Tracing) - uncompressed	109.73 KB	added	added
CDN Bundle (incl. Tracing, Replay) - uncompressed	222.17 KB	added	added
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	235.38 KB	added	added
⛔️ @sentry/nextjs (client) (max: 38.09 KB)	38.17 KB	added	added
@sentry/sveltekit (client)	35.74 KB	added	added
@sentry/node	124.95 KB	added	added
@sentry/node - without tracing	94.01 KB	added	added
@sentry/aws-serverless	103.61 KB	added	added