Unpin dependencies after bumping MSRV beyond 1.72

[goatgoose]

Problem:

The zeroize dependency of aws-lc-rs -> s2n-tls-sys has updated their MSRV to 1.72 in v1.8.0. This breaks our build, since our MSRV is < 1.72. The zeroize dependency will be pinned to work around this MSRV increase.

Solution:

zeroize should be unpinned after our MSRV is increased to 1.72.

[dougch]

also #4395