Bump base64 from 0.21.5 to 0.21.6

Bumps [base64](https://github.com/marshallpierce/rust-base64) from 0.21.5 to 0.21.6.
- [Changelog](https://github.com/marshallpierce/rust-base64/blob/master/RELEASE-NOTES.md)
- [Commits](marshallpierce/rust-base64@v0.21.5...v0.21.6)

---
updated-dependencies:
- dependency-name: base64
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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