Enhance sast-rule csharp/xss/rule-Xss.yml

Problem

• There are many suspect sinks such as:

- pattern: "(System.Web.HttpResponse $E).AddHeader(...)"
- pattern: "(System.Web.HttpResponse $E).AppendHeader(...)"

• Determine valid sinks for XSS
• Consider if this rule should be split into multiple XSS rules

Solution

Follow the enhance rule checklist.