qmanager: track API changes in core

Problem: the raw RPC interfaces are changing to use a size_t
instead of int for payload size.

Update scheduler usage.

flux-framework/flux-core#6467 must be merged before this will pass CI.

qmanager/modules/qmanager.cpp

@@ -280,7 +280,7 @@ static int handshake_jobmanager (std::shared_ptr<qmanager_ctx_t> &ctx)
 
 static void status_request_cb (flux_t *h, flux_msg_handler_t *w, const flux_msg_t *msg, void *arg)
 {
-    int len = 0;
+    size_t len = 0;
     const char *payload;
     flux_future_t *f = NULL;
 
@@ -310,7 +310,7 @@ static void feasibility_request_cb (flux_t *h,
                                     const flux_msg_t *msg,
                                     void *arg)
 {
-    int size = 0;
+    size_t size = 0;
     flux_future_t *f = nullptr;
     const char *data = nullptr;