Update security issue reporting instructions

[danroth27]

Update security issue reporting instructions to match dotnet/runtime:

• https://github.com/dotnet/runtime/blob/main/README.md#reporting-security-issues-and-security-bugs
• https://github.com/dotnet/runtime/blob/main/SECURITY.md

Fixes #64393

[Copilot]

Pull request overview

This PR updates the security issue reporting instructions in SECURITY.md and README.md to align with the dotnet/runtime repository's approach. The changes modernize the reporting process by directing users to the MSRC Researcher Portal instead of email-based reporting, and remove outdated references to PGP keys.

Key changes:

• Replaces email-based security reporting (secure@microsoft.com) with the MSRC Researcher Portal web form
• Updates FAQ links from the old TechCenter to current MSRC FAQ pages
• Adds cross-reference between README.md and SECURITY.md for better documentation navigation
• Includes reference to the Microsoft .NET Bounty Program in README.md

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
SECURITY.md	Updates vulnerability reporting method from email to MSRC Researcher Portal, modernizes FAQ link, removes PGP key reference
README.md	Updates security reporting instructions to match SECURITY.md, adds cross-reference to SECURITY.md, adds bounty program link