RPC changes
-----------
The RPC `getwalletinfo` response now includes the `scanning` key with an object
if there is a scanning in progress or `false` otherwise. Currently the object
has the scanning duration and progress.

Add release notes for 15730
doc: Add release notes for 15730

Former-commit-id: 9b04a82bda2924c031750def04806f4698c36c02