Breaking: #19737 - Prefer root templates for pages

Description

The Core now gives templates having set the root-flag precedence over normal extension templates. This ignores the fact that normally the first template of the current page is chosen, following their sorting.

This improves user experience as the user expects the root template to be the one with highest priority.

Impact

Pages where multiple templates are present not having a root-template as the topmost template in the list will encounter different results when templates are evaluated.

Affected installations

Installations with pages where multiple templates are present not having a/the root-template as the topmost template.

Migration

Ensure the templates have correct flags set.