WordPress dependencies update

[eugene-manuilov]

Summary

Addresses issue #1356

Relevant technical choices

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

[felixarntz]

@eugene-manuilov I've downgraded React as part of this one back to 16.12.0 due to this error when opening a Site Kit module's settings that only surfaces on 16.13+:

Warning: Cannot update a component (`%s`) while rendering a different component (`%s`). To locate the bad setState() call inside `%s`, follow the stack trace as described in https://fb.me/setstate-in-render%s PermissionsModal AdBlockerWarning AdBlockerWarning 
        in AdBlockerWarning
        in Unknown
        in InnerComponent (created by WithFilters(ModuleSettingsWarning))
        in WithFilters(ModuleSettingsWarning)
        in div
        in SetupModule
        in SettingsModule
        in div
        in div
        in div
        in div
        in Layout
        in div
        in SettingsModules
        in div
        in div
        in div
        in SettingsApp
        in RestoreSnapshots
        in ErrorHandler
        in Root

We can investigate this problem via a separate issue, for now let's make sure we can wrap up this initial bulk dependency updates issue.

I've also done another minor WP dependency update since those new versions were literally published as I was reviewing the PR.