Upgrade 'tokio-tungstenite' to 0.20.

[BlackDex]

Cherry-Pick tungstenite update to v0.5 branch.
This ensures applications which use the contrib/ws to use a patched version which has a known CVE https://github.com/snapview/tokio-tungstenite/blob/master/CHANGELOG.md#0201

[SergioBenitez]

As noted on Matrix, the vulnerability in tungstenite doesn't affect rocket_ws as we don't make use of its HTTP handshaking functionality.

[SergioBenitez]

I just finished cherry picking every possible commit from master to 0.5-rc, which included this commit. I preferred to do it this way to keep the commits in relative order.

[BlackDex]

Thx!