fix invalid bitSize in strconv.ParseFloat (#40753)

The bitSize argument in strconv.ParseFloat should be either 32 or 64.
Any value other than 32 will default to 64 in the current implementation,
though this behavior is undocumented, making it undefined.

See https://cs.opensource.google/go/go/+/refs/tags/go1.23.0:src/strconv/atof.go;l=704-708.

libbeat/processors/ratelimit/limit.go

@@ -46,7 +46,7 @@ func (l *rate) Unpack(str string) error {
 	valueStr := strings.TrimSpace(parts[0])
 	unitStr := strings.TrimSpace(parts[1])
 
-	v, err := strconv.ParseFloat(valueStr, 8)
+	v, err := strconv.ParseFloat(valueStr, 64)
 	if err != nil {
 		return fmt.Errorf(`rate's value component is not numeric: %v`, valueStr)
 	}