Release v0.7.0

Release v0.7.0-alpha.1

chore: prepare v0.6.21 release (tokio-rs#1173)

This removes the `=` dependency on an older `cfg-if` version.

prepare v0.6.20 (tokio-rs#1163)

Bump version to v0.6.19 (tokio-rs#961)

Bump version to v0.6.18 (tokio-rs#954)

Bump version to v0.6.17 (tokio-rs#942)

Bump version to v0.6.16 (tokio-rs#876)

Bump version to v0.6.15 (tokio-rs#854)

Bump version to v0.6.14 (tokio-rs#826)