Clean up whats_new_placement experiment

Summary

Productizes the winning variant of the whats_new_placement A/B experiment: the "What's new for you" item (compass icon) now always shows in the Profile/user menu, and the experiment scaffolding (experiment class, spec, feature flag definition, and the publish call in EE::SidebarsHelper) is removed.

Docs

• doc/development/experiment_guide/testing_experiments.md and doc/development/internal_analytics/capturing_snowplow_events_in_specs.md no longer name a live experiment; they use an illustrative my_experiment name instead.
• spec/support_specs/helpers/snowplow_tracking_journey_spec.rb now writes its own fixtures to a temp dir instead of loading real journeys, so future experiment cleanups don't need to touch it.
• The whats_new_placement journey fixture under spec/fixtures/snowplow_tracking_journeys/ is deleted rather than converted to a plain journey: every event it declared was experiment-only and is removed here, and the menu item click uses the generic click_button event, while opening the drawer fires the new open_whats_new_drawer event.

Behavior changes to note

• Self-managed users move from the Help menu item to the Profile menu item without ever having seen the experiment, since it only ran on GitLab.com.
• The drawer open event is now tracked as open_whats_new_drawer instead of click_whats_new_drawer; the WhatsNewUsage Tableau dashboard currently filters on click_whats_new_drawer and will need to be updated to open_whats_new_drawer once this ships.
• Logged-out visitors have no user menu, so the Help menu still shows "What's new for you" for them (keyed off the existing is_logged_in sidebar flag). Signed-in users see it only in the Profile menu.

Validation steps

1. On GDK, sign in.
2. Open the Profile menu (click your avatar) and confirm "What's new for you" shows with the compass icon.
3. Click it and confirm the What's new drawer opens.
4. Open the ? Help menu and confirm "What's new for you" is not there while signed in.
5. Repeat steps 1-4 with GITLAB_SIMULATE_SAAS=1 set.
6. Sign out and confirm the Help menu shows "What's new for you" and clicking it opens the drawer.
7. Optionally, using Snowplow Micro or the browser network tab, confirm open_whats_new_drawer fires with property: profile_menu when signed in and property: help_menu when signed out, the menu item click fires click_button with label whats_new, and no whats_new_placement experiment context is attached.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab/-/issues/605715+s