Bump base64 from 0.21.5 to 0.21.6 in /rust

[dependabot[bot]]

Bumps base64 from 0.21.5 to 0.21.6.

Changelog

Sourced from base64's changelog.

0.21.6

• Improved introductory documentation and example

Commits

• 1cd75a1 v0.21.6
• 5820d9b Merge pull request #263 from marshallpierce/silverlyra-usage
• 97d9180 Merge branch 'usage' of github.com:silverlyra/rust-base64 into silverlyra-usage
• b64c624 Merge pull request #262 from tamird/clap
• e32ca18 examples: replace structopt with clap
• 72f25a8 Simplify the example program
• 40e162c Rewrite the crate docstring
• bfde751 Merge pull request #260 from marshallpierce/mp/doc-grammar
• 5466e43 Minor doc improvements
• baffe6c Merge pull request #259 from marshallpierce/serprex-master
• Additional commits viewable in compare view

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