chore: Add Cargo.lock, fix builds on 1.75 (#1942)

[icu4x](https://github.com/unicode-org/icu4x) bumped its MSRV to 1.81
for all of it's sub-crates in a minor release. This means that we can
[no longer build hugr with
1.75](https://github.com/CQCL/hugr/actions/runs/13569763210/job/37931524942#step:7:315).

This PR unignores the `Cargo.lock` and `cargo update`s the dependencies
to downgrade them to a supported version. We should see the 1.75 CI job
fail if dependabot tries to update them again.

This problem is caused by the dependency resolver v2 ignoring the
`rust-version` field in the crates. In rust 2024 the new v3 resolver
should be able to use only valid versions, but we have a long way until
we are able to use that.

.github/change-filters.yml

@@ -3,13 +3,16 @@
 
 model: &model
   - "hugr-model/**"
+  - "Cargo.lock"
+  - "Cargo.toml"
 
 rust: &rust
   - "hugr/**"
   - "hugr-cli/**"
   - "hugr-core/**"
   - "hugr-passes/**"
   - *model
+  - "Cargo.lock"
   - "Cargo.toml"
   - "specification/schema/**"
   - ".github/workflows/ci-rs.yml"

.gitignore

@@ -3,10 +3,6 @@
 debug/
 target/
 
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
-
 # These are backup files generated by rustfmt
 **/*.rs.bk