Track Wordpress 5.5 & 5.6 breaking changes and upgrade path for p4

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