build(deps): bump regex from 1.5.5 to 1.5.6 in /atcoder-problems-backend

Bumps [regex](https://github.com/rust-lang/regex) from 1.5.5 to 1.5.6.
- [Release notes](https://github.com/rust-lang/regex/releases)
- [Changelog](https://github.com/rust-lang/regex/blob/master/CHANGELOG.md)
- [Commits](rust-lang/regex@1.5.5...1.5.6)

---
updated-dependencies:
- dependency-name: regex
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]