chore(clippy): deny indexing at workspace-level

Cargo.toml

@@ -102,5 +102,5 @@ unused_must_use = "deny"
 
 [workspace.lints.clippy]
 # Clippy lints are documented here: https://rust-lang.github.io/rust-clippy/master/index.html
-indexing_slicing = "warn"
+indexing_slicing = "deny"
 missing_safety_doc = { level = "warn" }