Flag stable rustc/clippy warnings in CI

Today, when a change introduces warnings, the change author might not see
them. Fix that by making clippy fail on warnings.

AFAICT, "clippy --deny-warnings" will also fail on rustc warnings.
I'd imagine this is what most respectable Rust projects do.

Pin stable rust so we won't get unrelated failures. Alternatively, we could
keep using "dtolnay/rust-toolchain@stable", that should be fine too (as long
as we have the capacity to quickly silence/iron out clippy failures).

While at it, remove some unneeded dependencies. Keep gettext because that
one might enable some cfg-directives (?).
Other cfgs like feature="benchmark" and target_os != "linux" are not yet checked in CI.

See #11584

.github/actions/rust-toolchain@stable/action.yml

@@ -16,7 +16,7 @@ permissions:
 runs:
   using: "composite"
   steps:
-    - uses: dtolnay/rust-toolchain@stable
+    - uses: dtolnay/rust-toolchain@1.88
       with:
         targets: ${{ inputs.targets }}
         components: ${{ inputs.components }}