rust-toolchain.toml and MSRV checks in CI

[eamsden]

What was changed

Add MSRV on temporalio-core-sdk crate, and add CI step to use cargo msrv to check it.

Why?

We want to have a rust-toolchain set to keep us up to date with the latest tooling, but also make sure that our published MSRV (a) exists and (b) is accurate.

Checklist

• Verify CI working
• remove feature branch from workflow push field after testing.
• Check that installing cargo-msrv and building with an old toolchain version (via cargo msrv verify) don't break the Rust caching in CI. (made sure it doesn't by using a separate rust-cache key)

1. Closes

2. How was this tested:

• Repeated pushes to github running CI
• cargo check locally.

3. Any docs updates needed?
   No

[CLAassistant]

All committers have signed the CLA.

[semgrep-managed-scans]

Semgrep found 1 missing-explicit-permissions finding:

• .github/workflows/heavy.yml
  • L15-28 - Triage

No explicit GITHUB_TOKEN permissions found at the workflow or job level. Add a permissions: block at the workflow root (applies to all jobs) or per job with least privilege (e.g., contents: read and only specific writes like pull-requests: write if needed).

[claude]

Claude Code Review

This repository is configured for manual code reviews. Comment @claude review to trigger a review.

_{Tip: disable this comment in your organization's Code Review settings.}

[Sushisource]

Sweet!