Simplify String concatenation

Closes gh-23470

spring-core/src/main/java/org/springframework/util/backoff/FixedBackOff.java

@@ -109,14 +109,12 @@ public class FixedBackOff implements BackOff {
 
 		@Override
 		public String toString() {
-			final StringBuilder sb = new StringBuilder("FixedBackOff{");
-			sb.append("interval=").append(FixedBackOff.this.interval);
 			String attemptValue = (FixedBackOff.this.maxAttempts == Long.MAX_VALUE ?
 					"unlimited" : String.valueOf(FixedBackOff.this.maxAttempts));
-			sb.append(", currentAttempts=").append(this.currentAttempts);
-			sb.append(", maxAttempts=").append(attemptValue);
-			sb.append('}');
-			return sb.toString();
+			return "FixedBackOff{interval=" + FixedBackOff.this.interval +
+					", currentAttempts=" + this.currentAttempts +
+					", maxAttempts=" + attemptValue +
+					'}';
 		}
 	}