Remove request postData and response content

What does this MR do?

Removes the response.content and request.postData values from Alert. This ensures the data can be garbage collected as these values can have large amounts of data.

request.postData will contain the HTTP body of a requested that was POSTed to a URL. There is no theoretical limit to this size.

response.content will contain the HTTP body of a response from any GET/POST/PUT/HEAD/etc call to a URL. There is no theoretical limit to this size, and we know this will be large when files can be downloaded.

What are the relevant issue numbers?

gitlab-org/gitlab#225270 (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
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer