feat(core)!: Pass root spans to beforeSendSpan and disallow returning null

[chargome]

• Disallows returning null from beforeSendSpan
• Passes root spans to beforeSendSpan
• Adds entry to migration guide and changelog

Looked at two options for passing the root span:

1. Within the SentrySpan class, before converting the root span into a transaction.
2. Parsing a SpanJSON out of an event and then writing back the updated values into the event.

Went with (2), passing the root span within processBeforeSend to have all processing hooks centrally in one place and because the other approach would have been too messy.

closes #14336

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.31 KB	+2.04%	+476 B 🔺
@sentry/browser - with treeshaking flags	21.98 KB	+1.84%	+405 B 🔺
@sentry/browser (incl. Tracing)	35.79 KB	+1.05%	+380 B 🔺
@sentry/browser (incl. Tracing, Replay)	73.08 KB	+0.51%	+376 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	63.44 KB	+0.54%	+344 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas)	77.47 KB	+0.47%	+371 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback)	89.84 KB	+0.4%	+365 B 🔺
@sentry/browser (incl. Feedback)	40.05 KB	+1.12%	+454 B 🔺
@sentry/browser (incl. sendFeedback)	27.92 KB	+1.58%	+444 B 🔺
@sentry/browser (incl. FeedbackAsync)	32.68 KB	+1.35%	+445 B 🔺
@sentry/react	26.05 KB	+1.69%	+441 B 🔺
@sentry/react (incl. Tracing)	38.63 KB	+1.01%	+394 B 🔺
@sentry/vue	27.49 KB	+1.36%	+376 B 🔺
@sentry/vue (incl. Tracing)	37.61 KB	+0.9%	+340 B 🔺
@sentry/svelte	23.46 KB	+1.91%	+449 B 🔺
CDN Bundle	24.59 KB	+1.51%	+373 B 🔺
CDN Bundle (incl. Tracing)	36.13 KB	+1.03%	+374 B 🔺
CDN Bundle (incl. Tracing, Replay)	71.25 KB	+0.52%	+374 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback)	76.47 KB	+0.51%	+395 B 🔺
CDN Bundle - uncompressed	72.12 KB	+1.58%	+1.12 KB 🔺
CDN Bundle (incl. Tracing) - uncompressed	107.54 KB	+1.01%	+1.07 KB 🔺
CDN Bundle (incl. Tracing, Replay) - uncompressed	221.78 KB	+0.49%	+1.07 KB 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	234.65 KB	+0.46%	+1.07 KB 🔺
@sentry/nextjs (client)	38.92 KB	+1%	+392 B 🔺
@sentry/sveltekit (client)	36.34 KB	+1.11%	+406 B 🔺
@sentry/node	163.28 KB	+0.2%	+319 B 🔺
@sentry/node - without tracing	99.03 KB	+0.3%	+297 B 🔺
@sentry/aws-serverless	128.89 KB	+0.25%	+318 B 🔺

View base workflow run

[Lms24]

This generally looks good to me and I think the conversion is correct. Before we merge this, let's please add a sentence to the PR description why we went with this approach instead of calling beforeSendSpan earlier for posterity.