Replace deprecated getPastTimeString with getTimeSpanString (#1007)

Replaced with OpenRewrite command:

mvn -U org.openrewrite.maven:rewrite-maven-plugin:run \
      -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-jenkins:RELEASE \
      -Drewrite.activeRecipes=org.openrewrite.jenkins.migrate.hudson.UtilGetPastTimeStringToGetTimeSpanString

jenkinsci/jenkins#4174 deprecates the API in a
2019 release of Jenkins core.

https://javadoc.jenkins.io/hudson/Util.html#getPastTimeString(long)
declares it deprecated.

src/main/java/com/nirima/jenkins/plugins/docker/DockerDisabled.java

@@ -120,7 +120,7 @@ public String getWhenDisabledBySystemString() {
         final long now = readTimeNowInNanoseconds();
         final long howLongAgoInNanoseconds = now - nanotimeWhenDisabledBySystem;
         final long howLongAgoInMilliseconds = TimeUnit.NANOSECONDS.toMillis(howLongAgoInNanoseconds);
-        return Util.getPastTimeString(howLongAgoInMilliseconds);
+        return Util.getTimeSpanString(howLongAgoInMilliseconds);
     }
 
     /**