Add new fee structure with all sub-fields denominated in BTC

doc/release-notes.md

@@ -70,6 +70,10 @@ RPC changes
   /rest/block/ endpoints when in json mode. This is also included in `getblock`
   (with verbosity=2), `listsinceblock`, `listtransactions`, and
   `getrawtransaction` RPC commands.
+- New `fees` field introduced in `getrawmempool`, `getmempoolancestors`, `getmempooldescendants` and
+   `getmempoolentry` when verbosity is set to `true` with sub-fields `ancestor`, `base`, `modified`
+   and `descendent` denominated in BTC. This new field deprecates previous fee fields, such as
+   `fee`, `modifiedfee`, `ancestorfee` and `descendentfee`.
 
 External wallet files
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