Why this change?
Back in May 17 2023 along with the release of Discourse 3.1, we announced
on meta that the legacy hamburger dropdown navigation menu is
deprecated and will be dropped in Discourse 3.2. This is the link to the announcement
on meta: https://meta.discourse.org/t/removing-the-legacy-hamburger-navigation-menu-option/265274
## What does this change do?
This change removes the `legacy` option from the `navigation_menu` site
setting and migrates existing sites on the `legacy` option to the
`header dropdown` option.
All references to the `legacy` option in code and tests have been
removed as well.
This new site setting replaces the
`enable_experimental_sidebar_hamburger` and `enable_sidebar` site
settings as the sidebar feature exits the experimental phase.
Note that we're replacing this without depreciation since the previous
site setting was considered experimental.
Internal Ref: /t/86563
