Bump minimum required WordPress version

[acicovic]

Is your feature request related to a problem?

Our plugin claims to work on WordPress 5.2 and later. This is true for the plugin's core, but the Content Helper and our React components have been receiving numerous updates which might have broken things.

Describe the solution you'd like

Test the PCH on a WordPress 5.2 installation. If it doesn't work, take any necessary actions to fix it or to bump the minimum WordPress version.

[acicovic]

After completing our research, we are gravitating towards setting WordPress 6.3 as the minimum required WordPress version in our README.md.

Many popular Automattic plugins require WordPress 6.3 or newer (Jetpack, Gutenberg, WooCommerce), as do other 5-million+ install plugins (Yoast, Contact Form 7). Requiring WP 6.3 doesn't seem to be a threat to their popularity. WordPress 6.3.0 was released on 8 August 2023.

The detailed test results of the PCH against WordPress versions are as follows:

• WP 6.5.2, 6.4.4, 6.3.4: Works as expected.
• WP 6.2.5: Crashes the Post Editor when the PCH Sidebar icon is clicked.
• WP 6.1.6: Works, though PCH Sidebar tabs are tall and without icons.
• WP 6.0.8: The Dashboard Widget doesn’t work. The plugin’s Sidebar doesn’t appear and a The "wp-parsely-block-editor-sidebar" plugin has encountered an error and cannot be rendered. error is displayed.
• WP 5.9.9, 5.8.9: The Dashboard Widget doesn’t work. The Post Editor crashes completely on load.

[acicovic]

We currently use libxml, which should be fine for our current requirement of PHP 7.2 and up. As we're planning for upcoming features, we might keep libxml or choose to use one of the following:

• WP_HTML_Tag_Processor: Introduced in WP 6.2, so this is fine to use.
• WP_HTML_Processor: Introduced in WP 6.4, which is discussable since it's a minor bump above WP 6.3 (our original intent) and as WP 6.6 launches soon.

So both seem viable options, unless we need to use functionality that was introduced after their initial versions.