cargo: Update Rust crate hashbrown to 0.15.3

See merge request gitlab-org/ruby/gems/prometheus-client-mmap!225

This pins the versions used to build this gem and avoids floating
dependencies as we saw when rb_sys was updated.

We made a similar change in
gitlab-glfm-markdown!87.

Add Gemfile.lock and make cache dependent on it

See merge request !231

Cargo.lock is already using 0.9.115.

Update Cargo.toml to rb-sys 0.9.115

See merge request !233

This came out of
gitlab-org/gitlab-development-kit!5120.

Test with Rust 1.89.0

See merge request !239

Ruby 4 compat

See merge request !243

Release v1.3.0

See merge request !244