Fix issuable type comparison in IssuableExportCsvWorker

What does this MR do?

In !52706 (merged) we added to IssuableExportCsvWorker a behaviour exclusive to Requirements but it's not working as expected because the issuable_type argument will be a string and it's currently being compared with a symbol.

This MR convert the type to a symbol before comparing it with :requirement. The frontend changes are not implemented yet so to reproduce we can try the migration with selectedFields argument like

mutation {
  exportRequirements(input: {projectPath: "group-1/project-1", selectedFields: ["title", "description"]}) {
    errors
  }
}

The CSV file emailed to the user will contain all headers instead of Title and Description only.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #290823 (closed)