Navigation: Add a link to update WP

[psealock]

This is a follow up to #5753 (comment).

Navigation will only be available to users with at least WP 5.6.0 installed because of the availability of Gutenberg component Navigation after wp.components is unbundled. When trying to enable the Navigation feature on an older version of WP users should get this link:

Deliverables

• Users on WP < 5.6.0 see a link to update WP versions on the Feature setting.
• Users on WP < 5.6.0 see a disabled checkbox on the Feature setting.
• Users on WP >= 5.6.0 see no link to update and the checkbox is enabled

cc @elizaan36