Add weekly security-focused cargo update workflow for release branches

[tillrohrmann]

Summary

Automate weekly cargo update PRs against release branches to pick up security patches in Rust dependencies. This complements the Docker image refresh workflow (#4330) by addressing dependency updates that can't be fixed by base image updates alone.

Motivation

Some dependencies, like the vendored OpenSSL in librdkafka, won't be updated by refreshing the base Docker image. A weekly cargo update workflow would help ensure security patches in Rust dependencies are picked up promptly.

Proposed Implementation

• Create a GitHub Actions workflow that runs weekly
• Target the latest release branch (e.g., release-1.6)
• Run cargo update and create a PR if there are changes
• Could use tools like Dependabot or a custom workflow with cargo update

References

• Suggested by @pcholakov in Add weekly Docker image refresh workflow for security updates #4330
• Related: Docker image refresh workflow (Add weekly Docker image refresh workflow for security updates #4330)