chore(relay): Update default value from empty string to enabled (#64008)

`enabled` is the Relay default

src/sentry/options/defaults.py

@@ -869,7 +869,7 @@ register("relay.span-usage-metric", default=False, flags=FLAG_AUTOMATOR_MODIFIAB
 #
 # Note: To fully enable the cardinality limiter the feature `organizations:relay-cardinality-limiter`
 # needs to be rolled out as well.
-register("relay.cardinality-limiter.mode", default=None, flags=FLAG_AUTOMATOR_MODIFIABLE)
+register("relay.cardinality-limiter.mode", default="enabled", flags=FLAG_AUTOMATOR_MODIFIABLE)
 
 # Write new kafka headers in eventstream
 register("eventstream:kafka-headers", default=True, flags=FLAG_AUTOMATOR_MODIFIABLE)