[doc]: add 30275 release notes

doc/release-notes-30275.md

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+RPC
+---
+
+- The default mode for the `estimatesmartfee` RPC has been updated from `conservative` to `economical`.
+  which is expected to reduce overestimation for many users, particularly if Replace-by-Fee is an option.
+  For users that require high confidence in their fee estimates at the cost of potentially overestimating,
+  the `conservative` mode remains available.