program: update solana-security-txt

[2501babe]

copied the version macro from solana-program/stake#360 and fixed some old urls and syntax issues

query-security-txt ../target/deploy/spl_single_pool.so
Name: SPL Single-Validator Stake Pool
Project URL: https://spl.solana.com/single-pool

Contacts:
  Link: https://github.com/solana-program/single-pool/security/advisories/new
  Email: security@anza.xyz
  Discord: https://discord.gg/solana

Policy:
https://github.com/solana-program/single-pool/blob/main/SECURITY.md

Preferred Languages:
  en
Source code: https://github.com/solana-program/single-pool/tree/main/program
Source release: program@v4.0.1

Auditors:
  https://spl.solana.com/single-pool#security-audits

[joncinque]

Thanks!