refactor!: rename log feature to tracing-log

[mxinden]

Previously quinn* would provide the log feature to log events via log if no tracing subscriber exists.

Later https://github.com/quinn-rs/quinn/pull/1923 allowed quinn-udp to log via log directly, making tracing an optional dependency. For that, it introduced the direct-log feature, a workaround name in order to not introduce a breaking change.

This commit cleans up the above, renaming the log feature to tracing-log and the direct-log to log. This is a breaking change and thus quinn-udp is bumped to v0.6.0.

See https://github.com/quinn-rs/quinn/pull/1921 for the full history.

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Only merge before the next breaking change of quinn.

I am opening this up early as it is (a) still fresh in my mind, (b) in order to keep track of it and (c) make the next breaking change easier. Feel free to close if you don't think it is useful.