proxy: v2.105.0 (#4814)

This proxy release comprises mostly internal changes that set up for
upcoming discovery changes.

A `proxy_build_info` metric has been added to expose proxy build
metadata via the admin interface, i.e., for Prometheus.

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* ci: Run all builds on GitHub Actions (linkerd/linkerd2-proxy#604)
* error: Make backoff streams infallible (linkerd/linkerd2-proxy#605)
* trace: update tracing-subscriber to 0.2.8; add spans to JSON (linkerd/linkerd2-proxy#597)
* remove git deps on hyper and h2 (linkerd/linkerd2-proxy#596)
* Add proxy_build_info metric (linkerd/linkerd2-proxy#600)
* Move tls::accept to async/await (linkerd/linkerd2-proxy#607)
* Move metrics::Io to io::SensorIo (linkerd/linkerd2-proxy#610)
* Simplify proxy::Server as ServeHttp (linkerd/linkerd2-proxy#608)