ref(serverless): Use new span APIs for serverless

[AbhiPrasad]

Most of the changes here are with tests, otherwise we swap to use startInactiveSpan everywhere.

IMO it might be more correct to refactor to use startSpanManual, but that can come later, I want to keep same behavior for now.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.37 KB (0%)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.67 KB (0%)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.54 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.31 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.67 KB (0%)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.13 KB (0%)
@sentry/browser - Webpack (gzipped)	22.48 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	66.63 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.48 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.19 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	209.91 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	97.89 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	72.34 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	35.49 KB (0%)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.06 KB (0%)
@sentry/react - Webpack (gzipped)	22.52 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.68 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	49.81 KB (0%)
@sentry-internal/feedback - Webpack (gzipped)	17.12 KB (+0.02% 🔺)