bump version to v0.21.0 #25

Job	Run time
Test (x86_64-unknown-linux-gnu, ubuntu-latest, --features=native-tls)	1m 51s
Test (x86_64-unknown-linux-gnu, ubuntu-latest, --no-default-features --features=native-tls,online...	1m 45s
Test (x86_64-apple-darwin, macos-latest, --features=native-tls)	4m 23s
Test (x86_64-pc-windows-msvc, windows-latest, --features=native-tls)	3m 14s
Test (x86_64-unknown-linux-musl, ubuntu-latest, true)	2m 52s
Test (aarch64-unknown-linux-musl, ubuntu-latest, true, -- --test-threads=4)	3m 29s
Test (arm-unknown-linux-gnueabihf, ubuntu-latest, true, -- --test-threads=4)	3m 25s
Deploy (ubuntu-latest, aarch64-unknown-linux-musl, aarch64-linux-gnu, true)	3m 9s
Deploy (ubuntu-latest, arm-unknown-linux-gnueabihf, arm-linux-gnueabihf, true)	3m 0s
Deploy (ubuntu-latest, x86_64-unknown-linux-musl, true)	3m 36s
Deploy (macos-latest, x86_64-apple-darwin, --features=native-tls)	4m 56s
Deploy (windows-latest, x86_64-pc-windows-msvc, --features=native-tls, -C target-feature=+crt-sta...	4m 57s
	40m 37s

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