Update SLF4J and Logback Javadoc URL (#2229)
Update SLF4J and Logback Javadoc URL (#2229)

- www.slf4j.org and logback.qos.ch started to respond with non-200
  status.
line/armeria,trustin/armeria,trustin/armeria,minwoox/armeria,line/armeria,minwoox/armeria,anuraaga/armeria,line/armeria,trustin/armeria,kojilin/armeria,line/armeria,anuraaga/armeria,anuraaga/armeria,kojilin/armeria,anuraaga/armeria,trustin/armeria,anuraaga/armeria,minwoox/armeria,minwoox/armeria,anuraaga/armeria,minwoox/armeria,trustin/armeria,trustin/armeria,kojilin/armeria,kojilin/armeria,kojilin/armeria,line/armeria,minwoox/armeria,line/armeria,kojilin/armeria