Reintroduce FixedBackOff(long) constructor

Effectively removing FixedBackOff(Duration) only.

See gh-35028
See gh-35110

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

@@ -21,6 +21,7 @@ package org.springframework.util.backoff;
  * between two attempts and a maximum number of retries.
  *
  * @author Stephane Nicoll
+ * @author Sam Brannen
  * @since 4.1
  */
 public class FixedBackOff implements BackOff {
@@ -50,6 +51,17 @@ public class FixedBackOff implements BackOff {
 	public FixedBackOff() {
 	}
 
+	/**
+	 * Create an instance with the supplied interval and an unlimited number of
+	 * attempts.
+	 * @param interval the interval between two attempts in milliseconds
+	 * @since 7.0
+	 * @see #setMaxAttempts(long)
+	 */
+	public FixedBackOff(long interval) {
+		this.interval = interval;
+	}
+
 	/**
 	 * Create an instance with the supplied interval and maximum number of attempts.
 	 * @param interval the interval between two attempts in milliseconds