Settings: Legacy Forms: Add Warning Notice - #1164

Summary

Adds a non-dismissible yellow warning notice at Settings > Kit when a Post Type reference a legacy form.

Testing

• testWarningDisplayedForLegacyDefaultFormsSetting: Test a warning is displayed at Settings > Kit when a post type specifies a legacy form
• testWarningForLegacyPluginDefaultForm: Integration test to confirm get_legacy_warnings_for_plugin_settings returns a warning when a legacy form is specified in the Plugni settings.
• testNoWarningForPluginSettingsWithoutLegacyForms: Integration test to confirm get_legacy_warnings_for_plugin_settings returns no warnings when no legacy form is specified in the Plugin settings.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)