Prevent backslashes in meta content from being removed

[brianhogg]

Description

From the documentation of update_post_meta, if content we're trying to save contains backslashes, we should be calling wp_slash() to include them:

https://developer.wordpress.org/reference/functions/update_post_meta/#more-information

Fixes #2675

How has this been tested?

Manually

Screenshots

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 -->
• [x] My code has been tested.
• [ ] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.