fix: allow export of system defined and open taxonomies [FC-0036]

[rpenido]

Description

Small fix to allow api calls to EXPORT taxonomies with system_defined=True and allow_free_text=True.
The IMPORT restrictions where not changed.

---

Private-ref: FAL-3528

[openedx-webhooks]

Thanks for the pull request, @rpenido! Please note that it may take us up to several weeks or months to complete a review and merge your PR.

Feel free to add as much of the following information to the ticket as you can:

• supporting documentation
• Open edX discussion forum threads
• timeline information ("this must be merged by XX date", and why that is)
• partner information ("this is a course on edx.org")
• any other information that can help Product understand the context for the PR

All technical communication about the code itself will be done via the GitHub pull request interface. As a reminder, our process documentation is here.

Please let us know once your PR is ready for our review and all tests are green.

[openedx-webhooks]

@rpenido 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.