ci(renovate): stop dependency updates from bumping the go directive (#2951)

.github/renovate.json

@@ -50,6 +50,11 @@
       "matchManagers": ["gomod"],
       "matchDepTypes": ["golang"],
       "enabled": false
+    },
+    {
+      "description": "Skip releases whose own `go` directive exceeds ours. Without this, `gomodTidy` propagates their requirement into our go.mod and breaks the older Go versions in the CI matrix.",
+      "matchManagers": ["gomod"],
+      "constraintsFiltering": "strict"
     }
   ]
 }

experiments/errors.go

@@ -16,7 +16,7 @@ type InvalidValueError struct {
 
 func (err InvalidValueError) Error() string {
 	return fmt.Sprintf(
-		"task: Experiment %q has an invalid value %q (allowed values: %s)",
+		"task: Experiment %q has an invalid value %d (allowed values: %s)",
 		err.Name,
 		err.Value,
 		strings.Join(slicesext.Convert(err.AllowedValues, strconv.Itoa), ", "),