Bump subxt from 0.28.0 to 0.31.0

Bumps [subxt](https://github.com/paritytech/subxt) from 0.28.0 to 0.31.0.
- [Release notes](https://github.com/paritytech/subxt/releases)
- [Changelog](https://github.com/paritytech/subxt/blob/master/CHANGELOG.md)
- [Commits](paritytech/subxt@v0.28.0...v0.31.0)

---
updated-dependencies:
- dependency-name: subxt
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>