[PM-29067] Remove Feature Flag Code for PM-24032 (#20415)

* refactor(feature-flags): remove PM24032_NewNavigationPremiumUpgradeButton feature flag

* refactor(account-switcher): migrate to Angular Signals and OnPush change detection

* Revert "refactor(account-switcher): migrate to Angular Signals and OnPush change detection"

This reverts commit 0a48683be05267b4fb098de2842f766996fd1666.

* refactor(product-switcher): remove ConfigService from ProductSwitcherService

* test(product-switcher): update ProductSwitcherService tests after ConfigService removal