Bump cortex-m-rt from 0.7.3 to 0.7.4

Bumps [cortex-m-rt](https://github.com/rust-embedded/cortex-m) from 0.7.3 to 0.7.4.
- [Release notes](https://github.com/rust-embedded/cortex-m/releases)
- [Changelog](https://github.com/rust-embedded/cortex-m/blob/v0.7.4/CHANGELOG.md)
- [Commits](rust-embedded/cortex-m@v0.7.3...v0.7.4)

---
updated-dependencies:
- dependency-name: cortex-m-rt
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>