MM-62751: [Shared Channels] Allow remote users to be discoverable in the create DM/GM modal (#30918)

server/channels/db/migrations/postgres/000139_remoteclusters_add_last_global_user_sync_at.down.sql

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+ALTER TABLE remoteclusters DROP COLUMN IF EXISTS lastglobalusersyncat;

server/channels/db/migrations/postgres/000139_remoteclusters_add_last_global_user_sync_at.up.sql

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+ALTER TABLE remoteclusters ADD COLUMN IF NOT EXISTS lastglobalusersyncat bigint DEFAULT 0;