Avoid using deprecated mb_convert_encoding

[brianhogg]

Description

Avoids mb_convert_encoding warning with a video on a course.

Fixes #2672

How has this been tested?

Manually

Checklist:

• [x] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [ ] My code has been tested.
• [ ] My code passes all existing automated tests.
• [ ] My code follows the LifterLMS Coding & Documentation Standards.

[brianhogg]

Not all test cases pass: