Reinstate RedisCacheConfiguration.getTtl().

Reintroduce and deprecate RedisCacheConfiguration.getTtl for backwards compatibility.

Closes #2628
Original Pull Request: #2597

src/main/java/org/springframework/data/redis/cache/RedisCacheConfiguration.java

@@ -326,6 +326,26 @@ public class RedisCacheConfiguration {
 		return this.valueSerializationPair;
 	}
 
+	/**
+	 * Returns a computed {@link Duration TTL expiration timeout} based on cache entry key/value
+	 * if a {@link TtlFunction} was confiugred using {@link #entryTtl(TtlFunction)}.
+	 * <p>
+	 * Otherwise, returns the user-provided, fixed {@link Duration} if {@link #entryTtl(Duration)}
+	 * was called during cache configuration.
+	 *
+	 * @return the configured {@link Duration TTL expiration}.
+	 * @deprecated use {@link #getTtlFunction()} instead.
+	 */
+	@Deprecated
+	@SuppressWarnings("all")
+	public Duration getTtl() {
+
+		TtlFunction ttlFunction = getTtlFunction();
+
+		return ttlFunction instanceof FixedDurationTtlFunction it ? it.duration()
+			: ttlFunction.getTimeToLive(null, null);
+	}
+
 	/**
 	 * Gets the {@link TtlFunction} used to compute a cache key {@literal time-to-live (TTL) expiration}.
 	 *