Update string filter usage

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Updates FILTER_SANITIZE_STRING to FILTER_SANITIZE_FULL_SPECIAL_CHARS. The former is deprecated.

Similar to Automattic/newspack-ads#755

How to test the changes in this Pull Request:

Smoke test, should not result in any noticeable changes besides lack of a PHP Warning.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[dkoo]

Looks good! Aside: is there any way to enable/disable the Media Partners extension outside of calling Newspack\Settings::activate_optional_module( 'media-partners' ) via CLI?

[adekbadek]

Nope, there's no UI for this.

[github-actions]

Hey @adekbadek, good job getting this PR merged! 🎉

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[matticbot]

🎉 This PR is included in version 5.9.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 5.9.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀