fix(deps): update rust crate base64 to 0.21.7

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
base64	dependencies	patch	0.21.0 -> 0.21.7

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Release Notes

marshallpierce/rust-base64 (base64)

v0.21.7

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

v0.21.6

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• Improved introductory documentation and example

v0.21.5

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• Add Debug and Clone impls for the general purpose Engine

v0.21.4

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• Make encoded_len const, allowing the creation of arrays sized to encode compile-time-known data lengths

v0.21.3

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• Implement source instead of cause on Error types
• Roll back MSRV to 1.48.0 so Debian can continue to live in a time warp
• Slightly faster chunked encoding for short inputs
• Decrease binary size

v0.21.2

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• Rollback MSRV to 1.57.0 -- only dev dependencies need 1.60, not the main code

v0.21.1

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• Remove the possibility of panicking during decoded length calculations
• DecoderReader no longer sometimes erroneously ignores
  padding #​226

Breaking changes

• Engine.internal_decode return type changed
• Update MSRV to 1.60.0

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