Upgrade existing sites to use AZ Navbar and deprecate theme setting

[djcelaya]

See Roadmap for navbars in Arizona Quickstart on the Main navigation docs page.

Conditions of satisfaction

• Existing AZQS3 sites should be upgraded to use AZ Navbar.
• Existing AZQS3 sites should not be adjusted to use 3 levels.
• The theme setting to enable/disable AZ Navbar should be deprecated.
• Profile-level install hooks introduced in Closes #5310: Enable AZ Navbar by default on new site installs #5312 to enable AZ Navbar on new builds only should be removed.

Question(s)

1. Is a post_update hook needed to upgrade existing sites to the new settings? Or is this redundant with a config distro update? Yes

[djcelaya]

From @trackleft on 2/20: https://www.drupal.org/about/core/policies/core-change-policies/how-to-deprecate#config-schema

[danahertzberg]

Prioritize communication/awareness within CWS on this. Look at communication for longer tail of sites (@camikazegreen @natebyerley @sanjanans)

[djcelaya]

We will not enforce 3rd level nav with this upgrade.