fix: remove unused dependency tracing-attributes

[mxinden]

Various quinn* crates depend on tracing. tracing in its default feature set includes tracing-attributes in order to expose #[instrument] procedural macro in tracing::instrument.

None of quinn* make use of the #[instrument] macro. Thus the tracing-attributes dependency is unused.

To remove the dependency from the tree, this commit:

1. Restricts tracing's features to std, effectively removing the attributes feature.
2. Consolidates the various tracing* imports across quinn* crates into workspace dependencies.

Let me know if you don't want to use workspace dependencies?

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Context:

• Mozilla is currently using NSPR to send and receive QUIC datagrams.
• There is an ongoing effort to replace NSPR with quinn-udp, see https://github.com/quinn-rs/quinn/issues/1749.
• This effort is becoming more concrete, see working implementation in Firefox and corresponding tracking issue.
• Each new Rust dependency needs to be audited. The less dependencies (e.g. here tracing-attributes) the better.

As always, thank you for this project!

[djc]

Very cool to see that quinn-udp is being considered for use in Firefox!