Revert regressions in global overrides

[burythehammer]

Changed

• Reverted global/regional override rework from v2.7.3 due to regressions. Active-Active database global configuration behavior has been restored to v2.7.2 state. Transit Gateway improvements from v2.7.3 have been preserved.
• rediscloud_active_active_subscription_database: Both global_enable_default_user and the region-level enable_default_user (in override_region blocks) now default to true. To disable the default user in a specific region, you must explicitly set enable_default_user = false in that region's override_region block.