wip update from 6.6.x

[leonstafford]

few considerations:

• cleanup txt files no longer used
• < PHP 7.3 users attempting to update/install/activate - should not get into error state, including not being able to delete, due to uninstall.php (which doesn't exist yet)
• folder deployment option removed, check issues when people have had that set as deploy method
• WP options name has changed, if 6.6.x detected in wp2static_options, migrate to statichtmloutput_options, ideally, removing any folder related options and setting selected deploy option to zip if folder was set

Breaking change notices for users/readme:

• deploy cache will be reset on update
• folder deployment option removed (use ZIP deployment, files will be generated and available in uploads/static-html-output/

Tasks

• [ ] verify update from 6.6.7 from wp.org (with non-folder deploy option)
• [ ] verify update from 6.6.7 from wp.org (with folder deploy option)
• [ ] verify update from previous dev build (range of 6.6.8 to 6.6.18)
• [ ] implement uninstall.php to clear settings and files on plugin deletion

[leonstafford]

This is very stale. Some useful stuff in here if wanting smooth 6.6.7 to 6.6.23 upgrade, but incomplete, so prefer to deal with possible user breakage for anyone on old version and focus on continued improvements on latest codebase.