Refactor site validation modal component

What does this MR do?

This MR

1. refactors the modal component (and corresponding GraphQL variables) to use fullPath instead of projectFullPath for consistency

2. removes the feature flag for HTTP Header Validation: It's no longer required given that there is another feature flag on top of that which would be rolled out in %13.7 via &4871 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry (the entire feature is behind a feature flag)

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