specs: add deposit tx gaslimit

specs/deposits.md

@@ -264,7 +264,7 @@ transaction are determined by the corresponding `TransactionDeposited` event emi
     - In case of a contract creation (cf. `isCreation`), this address is always zero.
 3. `mint` is set to the emitted value.
 4. `value` is set to the emitted value.
-5. `gaslimit` is unchanged from the emitted value.
+5. `gaslimit` is unchanged from the emitted value. It must be at least 21000.
 6. `isCreation` is set to `true` if the transaction is a contract creation, `false` otherwise.
 7. `data` is unchanged from the emitted value. Depending on the value of `isCreation` it is handled
    as either calldata or contract initialization code.