fix(deps): update rust crate base64 to 0.21.6

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
base64	dependencies	patch	0.21.5 -> 0.21.6

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Release Notes

marshallpierce/rust-base64 (base64)

### [`v0.21.6`](https://togithub.com/marshallpierce/rust-base64/blob/HEAD/RELEASE-NOTES.md#0216) [Compare Source](https://togithub.com/marshallpierce/rust-base64/compare/v0.21.5...v0.21.6) - Improved introductory documentation and example

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