Update Rust crate base64 to 0.21.7

This MR 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

Compare Source

• Support getting an alphabet's contents as a str via Alphabet::as_str()

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