Switch to minimum supported Rust version compatible resolver (#2239)

The url 2.5.4+ crate pulls idna 1.x -> idna_adapter -> icu_* 2.2.0,
which
require rustc 1.86. A fresh dependency resolution (e.g. the Docker
build,
which did not pass --locked) would select those and fail against the
1.85.0
toolchain pinned in rust-toolchain.toml.

Bump the workspace to resolver = "3". With no rust-version declared,
cargo's
MSRV-aware resolver falls back to the installed toolchain version
(1.85.0, per
rust-toolchain.toml) and resolves to the latest compatible deps. Also
pass
--locked in the Dockerfile so the image builds the exact pinned
dependency
tree.