feat(core): Use serialized spans in transaction event

[mydea]

Previously, we passed the span instance, which is a) tricky because it keeps references etc, and b) doesn't work with the new span schema anyhow (because nothing is readable off the spans).

This also means we can get rid of the SerializedEvent again, as this is now just the regular Event again.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.2 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.45 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.38 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	61.99 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.66 KB (-0.03% 🔽)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	32.66 KB (-0.03% 🔽)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31 KB (-0.06% 🔽)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31 KB (-0.06% 🔽)
@sentry/browser - Webpack (gzipped)	22.26 KB (-0.08% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75.35 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.01 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.85 KB (-0.06% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped)	23.83 KB (-0.08% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	210.31 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	99.04 KB (-0.01% 🔽)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	71.05 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	35.94 KB (-0.02% 🔽)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.73 KB (-0.02% 🔽)
@sentry/react - Webpack (gzipped)	22.29 KB (-0.08% 🔽)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.17 KB (-0.02% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	49.52 KB (-0.03% 🔽)
@sentry-internal/feedback - Webpack (gzipped)	17.07 KB (-0.11% 🔽)