Cargo: Update base64 to version 0.21

[ansasaki]

There were incompatible changes between version 0.13 and 0.21, which requires some changes in the code.

Supersedes #570

[codecov]

Codecov Report

Merging #575 (b2b1b89) into master (47e146e) will increase coverage by 0.06%.
The diff coverage is 95.65%.

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	58.90% <100.00%> (+0.06%)	⬆️
upstream-unit-tests	58.45% <71.42%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
keylime-agent/src/main.rs	35.80% <0.00%> (ø)
keylime-agent/src/crypto.rs	80.58% <100.00%> (+0.07%)	⬆️
keylime-agent/src/keys_handler.rs	65.22% <100.00%> (+0.40%)	⬆️
keylime-agent/src/quotes_handler.rs	51.08% <100.00%> (ø)
keylime-agent/src/serialization.rs	100.00% <100.00%> (ø)
keylime/src/tpm.rs	86.37% <100.00%> (ø)