Bump base64 to 0.21

[codefionn]

Bump base base64 dependency to 0.21.

Motivation

• Remove the possibility of panicking during decoded length calculations
• Some functions in base64 were deprecated

Solution

• Bump version of base64 to 0.21
• Fix code using the deprecated base64::encode and base64::decode functions

[jplatte]

Done in https://github.com/tower-rs/tower-http/pull/386, didn't notice this PR before writing mine. Sorry about the duplicated work.