Modal raises serverless KV store Dict limits to unlimited storage with new locking

Modal upgraded its serverless Dict key-value store: new Dicts now have unlimited storage instead of the previous 10GiB cap, and item expiry changed to 7 days since last write or read. A new skip_if_exists flag on .put() enables distributed locking and "exactly once" semantics, and Dicts now offer data durability. The changes apply only to newly created Dicts.