docs: release notes for dash#4023, 4704, 4756 (#4825)

* docs: add release notes for dash#4023

* docs: add release notes for dash#4704

* docs: add release notes for dash#4756

doc/release-notes-4023.md

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+Notable changes
+===============
+
+* Support for the BIP70 Payment Protocol has been dropped from Dash Qt. Interacting with BIP70-formatted URIs will return an error message informing them of support removal. The `allowselfsignedrootcertificates` and `rootcertificates` launch arguments are no longer valid.

doc/release-notes-4704.md

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+Updated RPCs
+------------
+
+* The `gettxoutsetinfo` RPC call now accept one optional argument (`hash_type`) that defines the algorithm used for calcluating the UTXO set hash, it will default to `hash_serialized_2` unless explicitly specified otherwise. `hash_type` will influence the key that is used to refer to refer to the UTXO set hash.

doc/release-notes-4756.md

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+Updated RPCs
+------------
+
+* The `generatedescriptor` RPC call has been introduced to allow mining a set number of blocks, defined by argument `num_blocks`, with the coinbase destination set to a descriptor, defined by the `descriptor` argument. The optional `maxtries` argument can be used to limit iterations.
+
+* The `generateblock` RPC call will mine an array of ordered transactions, defined by hex array `transactions` that can contain either transaction IDs or a hex-encoded serialized raw transaction and set the coinbase destination defined by the `address/descriptor` argument.
+
+* The `mockscheduler` is a debug RPC call that allows forwarding the scheduler by `delta_time`. This RPC call is hidden and will only be functional on mockable chains (i.e. primarily regtest). `delta_time` must be between 0 - 3600.