HTTP/2 cleartext (h2c) container probes

HTTP/2 cleartext (h2c) is widely used where TLS terminates at the edge while workloads speak plain HTTP/2 on the pod network (e.g. gRPC). The IETF HTTP/2 spec defines cleartext prior-knowledge connections, and mainstream HTTP libraries already support h2c. Kubernetes probes should speak that protocol instead of forcing a separate HTTP/1.1-only listener. This KEP adds a protocol field to the existing httpGet probe handler, allowing operators to select the HTTP version independently of the URI scheme. When protocol: HTTP2 is set (with the default scheme: HTTP), the kubelet performs the HTTP GET over HTTP/2 with prior knowledge instead of HTTP/1.1. Operators are no longer forced to run a second HTTP/1.1-only probe port or fall back to a tcpSocket probe that only confirms the socket is open rather than a valid HTTP-level response.