doc: Add legacy wallet removal release notes

doc/release-notes-32944-28710-32438-31250.md

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+# Legacy Wallet Removal
+
+BDB legacy wallets can no longer be created or loaded. They can be migrated to
+the new descriptor wallet format. Refer to the `migratewallet` RPC for more
+details.
+
+The legacy wallet removal drops redundant options in the bitcoin-wallet tool,
+such as `-withinternalbdb`, `-legacy`, and `-descriptors`. Moreover, the
+legacy-only RPCs `addmultisigaddress`, `dumpprivkey`, `dumpwallet`,
+`importaddress`, `importmulti`, `importprivkey`, `importpubkey`,
+`importwallet`, `newkeypool`, `sethdseed`, and `upgradewallet`, are removed.
+(#32944, #28710, #32438, #31250)

doc/release-notes-32944.md

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-# RPC
-
-`upgradewallet` has been removed. It was unused and only applied to unsupported legacy wallets.