Nav Unification: Update fallback menu for Jetpack sites

[mmtr]

Changes proposed in this Pull Request

Updates the fallback menu used for Jetpack sites so it's aligned with the latest changes we introduced for the dynamic menu in Automattic/jetpack#18921, in which we removed most of the submenus.

Before	After

Testing instructions

• Switch to a Jetpack site and go to Posts.
• Delete the IndexedDB database and reload the page.
• Make sure there are no submenus flashing up.

Part of #50225.

[matticbot]

Test live: https://calypso.live/?branch=update/nav-unification-jetpack-fallback

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Async-loaded Components (~7 bytes removed 📉 [gzipped])

name                                         parsed_size           gzip_size
async-load-calypso-my-sites-sidebar-unified       -488 B  (-0.3%)       -7 B  (-0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[tjcafferkey]

Tested and works as described! 🚀