Prepare 0.6.0 release

[rnestler]

Checklist

• [na] Tests added if applicable
• [na] README updated if applicable
• CHANGELOG updated if applicable
• [na] If a commit includes a breaking change, include the string [breaking-change] somewhere in the commit message

[rnestler]

We can't really do anything about the cargo audit fails, since it is an upstream issue. It doesn't make sense to delay the release because of it, since the old release is vulnerable as well. Also the issue is pretty low severity: https://rustsec.org/advisories/RUSTSEC-2021-0078

[g5pw]

Yeah, I agree. We should bump 0.6 out of the way and think about a good alternative to iron in #109 and make another release.

[dbrgn]

You could explicitly ignore that RUSTSEC issue in the CI config to get rid of the build failure.

Or mark it as allowed failure.