Bump Rust version to 1.81.0 #4336

Job	Run time
set-tags	5s
cargo-toml-feature-propagation	1m 54s
build	2m 15s
Check with rustfmt	16s
cargo-clippy	2m 14s
check-copyright	4s
toml-formatting	52s
typescript-formatting	21s
typescript-linting	15s
typescript-tests-flashbox	1s
rust-test	1s
typescript-tests	1s
typescript-dancebox-specs	1s
typescript-tests-frontier	1s
typescript-tests-relay	1s
check-api-augment	1s
typescript-tests-simple-template	1s
chopsticks-upgrade-test	1s
docker-tanssi	1s
zombienet-tests	1s
zombienet-test-upgrade	1s
zombienet-test-upgrade-containers	1s
	8m 29s

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