docs: fix dead links in the README

[luccasfraga]

Seven links in the README currently return 404. Six of them are Sentry blog posts that are still online — the blog dropped the date prefix from its URLs — and the seventh points at the old React Native site.

Every replacement below was checked and returns 200.

Old (404)	New (200)
blog.sentry.io/2021/03/11/performance-monitoring-support-for-react-native/	blog.sentry.io/performance-monitoring-support-for-react-native/
blog.sentry.io/2021/08/23/mobile-vitals-four-metrics-every-mobile-developer-should-care-about/	blog.sentry.io/mobile-vitals-four-metrics-every-mobile-developer-should-care-about/
blog.sentry.io/2022/02/22/tracking-stability-in-a-bluetooth-low-energy-based-react-native-app	blog.sentry.io/tracking-stability-in-a-bluetooth-low-energy-based-react-native-app
blog.sentry.io/2022/06/01/tips-for-optimizing-react-native-application-performance-part-1	blog.sentry.io/tips-for-optimizing-react-native-application-performance-part-1
blog.sentry.io/2022/06/28/tips-for-optimizing-react-native-application-performance-part-2-using-sentry	blog.sentry.io/tips-for-optimizing-react-native-application-performance-part-2-using-sentry
blog.sentry.io/2022/07/07/introducing-mobile-screenshots-and-suspect-commits	blog.sentry.io/introducing-mobile-screenshots-and-suspect-commits
facebook.github.io/react-native/blog/2019/07/03/version-60#native-modules-are-now-autolinked	reactnative.dev/blog/2019/07/03/version-60#native-modules-are-now-autolinked

Documentation only — no code, no public API surface touched.

[luccasfraga]

Heads-up on the red check: Changelog Preview is failing on every fork PR right now, not just this one — #6565 and #6550 show the same failure.

The job never gets as far as looking at the changelog. It fails at checkout:

Refusing to check out fork pull request code from a 'pull_request_target' workflow.
[...] set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.

actions/checkout now blocks checking out fork code under pull_request_target, so this is unrelated to the change here and can't be fixed from a contributor's side. Everything else on this PR is green.