doc: add release notes for explicit fee estimators and bumpfee change

doc/release-notes-11413.md

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+Updated or changed RPC
+----------------------
+
+The `bumpfee`, `fundrawtransaction`, `sendmany`, `sendtoaddress`, and `walletcreatefundedpsbt`
+RPC commands have been updated to include two new fee estimation methods "BTC/kB" and "sat/B".
+The target is the fee expressed explicitly in the given form. Note that use of this feature
+will trigger BIP 125 (replace-by-fee) opt-in.
+
+In addition, the `estimate_mode` parameter is now case insensitive for all of the above RPC commands.
+
+The `bumpfee` command now uses `conf_target` rather than `confTarget` in the options.