Migrate retrieving features to RTK

[kyle-ssg]

Description

The feature list store still uses legacy Flux patterns (AppActions.getFeatures/searchFeatures) to fetch features on several pages,
despite an existing RTK Query equivalent (useGetFeatureListQuery). Additionally, the Datadog widget page (WidgetPage.tsx) and its
dependencies (@datadog/ui-extensions-react, @datadog/ui-extensions-sdk) have been deprecated since 2025 and should be removed.

Current state

• FeatureExport.tsx, FeatureImport.tsx, and UserPage.tsx use AppActions.getFeatures/searchFeatures + FeatureListStore instead
  of RTK Query
• WidgetPage.tsx depends on deprecated Datadog UI extensions packages and is no longer used
• OrgEnvironmentSelect.tsx is only used by the widget page
• project-store.js getProject has duplicated force/non-force fetch branches left over from the getIsWidget guards

Objectives

• Migrate FeatureExport.tsx, FeatureImport.tsx, and UserPage.tsx from AppActions.getFeatures/searchFeatures to
  useGetFeatureListQuery
• Remove getFeatures and searchFeatures from app-actions.js and their GET_FLAGS/SEARCH_FLAGS dispatcher cases once no callers
  remain
• Delete WidgetPage.tsx, datadog-client.ts, useSetupCustomWidget.ts, OrgEnvironmentSelect.tsx, and the widget route
• Uninstall @datadog/ui-extensions-react and @datadog/ui-extensions-sdk
• Simplify project-store.js getProject by deduplicating the fetch branches