chore(ci): Update CI deps

[sou1118]

This pull request includes updates to the CI workflow and minor cleanup in the Rust code.

ubuntu-20.04 will delete, so I try to update CI dependencies.

Changes to CI workflow:

• .github/workflows/test.yml: Updated the workflow to trigger on push and pull_request events only for the main branch, added workflow_dispatch event, updated the runner to ubuntu-24.04, and upgraded the actions/checkout version to v4. Additionally, changed the installation command to use python3-protobuf and libnet1-dev.

Code cleanup:

• src/rust_criu_protobuf/rpc.rs: Removed the unnecessary #![allow(box_pointers)] attribute.

[sou1118]

Thanks!