fix(replay): ensure replay_id is not added to DSC if session expired

[mydea]

We noticed we still sometimes see "too long sessions", and it seems this is because of errors being tagged with a replay long after the session expired. After some digging, this may be because we do not check session expiration when adding the replay_id to the DSC. This PR fixes this.

I thought about adding a test for this, but feels this is a lot of work with all the timing issues etc., so I figure this is OK to do as-is, as this is pretty straightforward... WDYT?

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	82.67 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	71.78 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	30.94 KB (0%)
@sentry/browser - Webpack (gzipped)	21.26 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	73.03 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.93 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.09 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	233.84 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	87.77 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	62.76 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.71 KB (0%)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	83.05 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped)	21.29 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	99.44 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	47.83 KB (0%)