proxy: v2.126.0 (#5432)

This release introduces a new "opaque transport" feature that allows the
proxy to securely transport server-speaks-first and otherwise opaque TCP
traffic.

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* Remove the profiling tooling (linkerd/linkerd2-proxy#775)
* Enforce clippy lints (linkerd/linkerd2-proxy#774)
* opencensus: remove Tokio 0.2 deps (linkerd/linkerd2-proxy#776)
* trace: update task-tracking to Tokio 0.3 (linkerd/linkerd2-proxy#778)
* Ensure each crate can be built indpendently (linkerd/linkerd2-proxy#777)
* Apply clippy to all targets, including tests (linkerd/linkerd2-proxy#779)
* transport: expire idle TCP metrics (linkerd/linkerd2-proxy#781)
* Remove weights from endpoint metadata (linkerd/linkerd2-proxy#784)
* Ensure all forwarded TCP streams keep the proxy running (linkerd/linkerd2-proxy#786)
* Default the protocol detection timeout to 10s (linkerd/linkerd2-proxy#782)
* Transport opaque connections over mTLS (linkerd/linkerd2-proxy#785)
* identity: add a metric tracking when the proxy's cert expires (linkerd/linkerd2-proxy#787)
* identity: add `identity_cert_refresh_count` metric (linkerd/linkerd2-proxy#788)