proxy: v2.122.0 (#5279)

This release addresses some issues reported around clients seeing
max-concurrency errors by increasing the default in-flight request limit
to 100K pending requests.

Additionally, the proxy now sets an appropriate content-type when
synthesizing gRPC error responses.

---

* style: fix some random clippy lints (linkerd/linkerd2-proxy#749)
* errors: Set `content-type` for synthesized grpc errors (linkerd/linkerd2-proxy#750)
* concurrency-limit: Drop permit on readiness (linkerd/linkerd2-proxy#751)
* Increase the default buffer capacity to 100K (linkerd/linkerd2-proxy#752)
* Change default max-in-flight and buffer-capacity (linkerd/linkerd2-proxy#753)

.proxy-version

@@ -1 +1 @@
-v2.121.0
+v2.122.0