fix(tracing): Ensure we use s instead of ms for startTimestamp

[mydea]

Fixes a bug introduced in #7632 due to assuming timestampWithMs returns a timestamp in ms (which it does not do).

Also replaces all usage of timestampWithMs with timestampInSeconds.

Let's deprecate timestampWithMs in a follow up PR.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.04 KB (+0.01% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	65.66 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.59 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	58.12 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.19 KB (0%)
@sentry/browser - Webpack (minified)	69.07 KB (0%)
@sentry/react - Webpack (gzipped + minified)	21.21 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	49.05 KB (-0.02% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.62 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.85 KB (-0.02% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	45.35 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	39.28 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	64.29 KB (+0.02% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	57.26 KB (0%)

[jeengbe]

assuming timestampWithMs returns a timestamp in ms (which it does not do).

😄