Update vulnerability reporting docs to use direct email contact

[Copilot]

The security reporting guidance still directed users to GitHub private advisories, but that flow is not configured for this repository. This updates the documented reporting path to use the maintainer email address instead.

• Security policy

  • Replaced the GitHub private advisory instructions in SECURITY.md with a direct contact path: devrael1@protonmail.com
  • Added an explicit note that GitHub private vulnerability reporting is not configured for this repository

• Result

  • Contributors now have a working private disclosure channel
  • The documentation no longer points to a non-functional reporting mechanism

If you discover a security vulnerability in TorBoxSDK, please report it
responsibly by email at **devrael1@protonmail.com**.

GitHub's private security advisory reporting is not configured for this
repository, so please do not use it for vulnerability reports.

[Copilot]

Pull request overview

Updates TorBoxSDK’s security vulnerability reporting instructions to use a direct maintainer email address instead of GitHub private advisories, aligning the documentation with the repository’s current reporting setup.

Changes:

• Replaced GitHub private advisory reporting instructions with email-based reporting (devrael1@protonmail.com).
• Added an explicit note that GitHub private vulnerability reporting is not configured for this repository.