Remove documentation for GitLab WAF

Why are we doing this work

More information about deprecation: #271276 (closed)

We have to deprecate documentation for GitLab WAF and all related links/mentions.

Relevant links

• Epic

Non-functional requirements

• Documentation
• [-] Feature flag:
• [-] Performance:
• [-] Testing:

Implementation plan

• documentation update related Modsecurity documentation:
  • remove doc/user/project/clusters/protect/web_application_firewall
  • remove #### Web Application Firewall (ModSecurity) section from doc/user/clusters/applications.md,
  • remove links to Web Application Firewall and ## Web Application Firewall section from doc/user/application_security/threat_monitoring/index.md
  • remove ### Web Application Firewall (ModSecurity) customization section from doc/topics/autodevops/stages.md
  • remove AUTO_DEVOPS_MODSECURITY_SEC_RULE_ENGINE variable from doc/topics/autodevops/customize.md,
  • remove "ingress_modsecurity_enabled": true, from doc/development/usage_ping/index.md
  • remove links to - Web Application Firewall from doc/user/project/clusters/protect/index.md
• documentation remove reference to ModSecurity in .markdownlint.json
• documentation remove reference to ModSecurity in doc/.vale/gitlab/spelling-exceptions.txt