Update Rust crate base64 to 0.21.7

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
base64	dependencies	patch	0.21.6 -> 0.21.7

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Release Notes

marshallpierce/rust-base64 (base64)

### [`v0.21.7`](https://togithub.com/marshallpierce/rust-base64/blob/HEAD/RELEASE-NOTES.md#0217) [Compare Source](https://togithub.com/marshallpierce/rust-base64/compare/v0.21.6...v0.21.7) - Support getting an alphabet's contents as a str via `Alphabet::as_str()`

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