Ensure compatibility with WordPress 5.9

[dkotter]

Description of the Change

There was a reported issue with WP 5.9 where the Fix button was not working. This PR fixes this by updating the wp.data method we use to update the editor's content when using the block editor, as this method wasn't working in 5.9 anymore.

Also noticed a console error due to a missing key in our insecure asset list we output so fixed that as well.

We say we support down to WP 5.2 but in testing, things don't work on WP 5.2 but do work on WP 5.3+ (there is a minor styling issue with the Fix button in 5.3 and some console errors in 5.8 but everything seems to work).

Closes #63

Alternate Designs

None

Possible Drawbacks

None

Verification Process

Follow the steps listed on #63

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed - Ensure we support WP 5.9

Credits

Props @dkotter, @mohitwp

[peterwilsoncc]

This looks good to me, tested in block editor locally successfully.

Should INSECURE_CONTENT_VERSION and the plugin version be updated here or is that done as part of the merge to the production branch?

[jeffpaul]

@peterwilsoncc the version bumps happen during the release process, more on that here: https://github.com/10up/insecure-content-warning/blob/develop/CONTRIBUTING.md#release-instructions