Split the csharp/injection/rule-XXEInjection.yml based on object type

What does this MR do?

• Split the csharp/injection/rule-XXEInjection.yml rule with related test cases
  • Single rule for XMLDocument- csharp_injection_rule-XmlDocumentXXEInjection
  • Single rule for XmlReader - csharp_injection_rule-XmlReaderXXEInjection

What are the relevant issue numbers?

gitlab-org/gitlab#435926 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer