Review Mozilla 1972073: Convert Sidebar to config-based prefs

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1972073

Description

See !1913 (comment 3370281).

Upstream merged the Fluent message browser-layout-show-sidebar-desc into browser-layout-show-sidebar2 as part of switching to the config based approach.

In "BB 43902: Modify the new sidebar for Base Browser." we want to override the sidebar description with our existing browser-layout-show-sidebar-desc-limited string.

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