Add CVE-2021-42013 Apache HTTPD lab

[nomadY24]

Summary

• Added Docker Compose environment for CVE-2021-42013
• Added Apache HTTP Server 2.4.50 lab files
• Added README with vulnerability summary, environment setup, reproduction steps, PoC, risk score, and mitigation

Test

docker compose up --build -d
curl -i http://localhost:8083/
curl -i http://localhost:8083/cgi-bin/vuln.sh