Upgrade to Wordpress 5.6

Wordpress 5.5 was released!

Update: Wordpress 5.6 will be released on 8/12. Aiming to update directly to that, skipping 5.5.

Tasks

• Has builtin lazy-loading, so wait for PLANET-5297 first.
• Has xml sitemap support, so evaluate if our current plugin is still needed.
• Has blocks registry, so create a new action to disable it.
• Go through the field guide and see if other issues need action before upgrading. If so, open new tickets for bigger chunks of work and block this ticket.
• Check native Gutenberg blocks (eg. buttons) if work the same way.
• Test 5.6 in a test instance using a CI variable (WP_VERSION).
• When everything is ready update WP_VERSION variable at org-global context in CircleCI