refactor: migrate ENABLE_TEAMS off FEATURES-as-dict

`TeamsSearchIndexer.search_is_enabled()` read the flag via
`settings.FEATURES.get(cls.ENABLE_SEARCH_KEY, False)`, where
ENABLE_SEARCH_KEY is a class attribute holding the flag name
('ENABLE_TEAMS'). As with the other class-attribute indexers, the flag
name never appears as a quoted literal at the call site, so the earlier
reference grep missed this reader; the FeaturesProxy bridge kept it
working. The companion test already toggles the flag flat via
`@override_settings(ENABLE_TEAMS=False)`.

Read the dynamic key off flat settings with
`getattr(settings, cls.ENABLE_SEARCH_KEY, False)`. ENABLE_TEAMS is defined
in lms/envs/common.py (and openedx/envs/common.py).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>