doc: add release notes

Co-Authored-By: tdb3 <[email protected]>

doc/release-notes-31583.md

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+Updated RPCs
+---
+- `getmininginfo` now returns `nBits` and the current target in the `target` field. It also returns a `next` object which specifies the `height`, `nBits`, `difficulty`, and `target` for the next block.
+- `getdifficulty` can now return the difficulty for the next block (rather than the current tip) when calling with the boolean `next` argument set to true.
+
+New RPCs
+---
+- `gettarget` can be used to return the current target (for tip) or target for the next block (with the `next` argument)