docs: update Security section to direct disclosures (#84156)
This PR updates the README.md Security section:
- Removes reference to GitHub’s private vulnerability reporting feature.
- Directs researchers to email responsible.disclosure@vercel.com.
- Clarifies that researchers will be added to our Open Source Software
Bug Bounty program upon contacting us.
This change aligns our security disclosure process with Vercel’s current
bug bounty program enrollment flow.
