move to GET for internal stream READs instead of POST (#20160)

the main reason is to let Go net/http perform necessary
book keeping properly, and in essential from consistency
point of view its GETs all the way.

Deprecate sendFile() as its buggy inside Go runtime.

cmd/generic-handlers.go

@@ -251,7 +251,7 @@ func guessIsRPCReq(req *http.Request) bool {
 		return true
 	}
 
-	return req.Method == http.MethodPost &&
+	return (req.Method == http.MethodPost || req.Method == http.MethodGet) &&
 		strings.HasPrefix(req.URL.Path, minioReservedBucketPath+SlashSeparator)
 }