refactor(wallet)!: rename method to `last_unused_address` #126

Job	Run time
Check no_std	34s
Build and test (stable, true, --no-default-features --features miniscript/no-std)	1m 27s
Build & Test Examples (example_cli)	23s
Build and test (stable, true, --all-features)	1m 44s
Check WASM	1m 2s
Build and test (1.63.0, --no-default-features --features miniscript/no-std)	1m 44s
Build & Test Examples (example_bitcoind_rpc_polling)	26s
Build and test (1.63.0, --all-features)	1m 43s
Build & Test Examples (example_electrum)	45s
Rust fmt	7s
Build & Test Examples (example_esplora)	53s
Build & Test Examples (example_wallet_electrum)	47s
prepare	5s
Build & Test Examples (example_wallet_esplora_async)	58s
Build & Test Examples (example_wallet_esplora_blocking)	1m 7s
Build & Test Examples (example_wallet_rpc)	50s
clippy_check	27s
	15m 2s

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