chore: update safety notice on `keystore::Connection` #5582

Job	Run time
hack	6m 16s
check (fmt, --all -- --check)	33s
proteus-test	4m 44s
check (clippy, -- -D warnings)	3m 46s
proteus-wasm-test	3m 9s
check (clippy, -- -D warnings, --target wasm32-unknown-unknown)	1m 32s
check (check, --tests)	3m 28s
test	9m 26s
test-keystore-regressions	8m 7s
check (check, --tests, --target wasm32-unknown-unknown)	1m 27s
wasm-core-test	3m 8s
wasm-test (keystore)	1m 47s
wasm-test (mls-provider)	1m 52s
	49m 15s

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