RSGroupBalancer#assignments lost some regionPlans

https://github.com/apache/hbase/blob/b2f2c79d8fa18fb691e669419004cc5168b0838d/hbase-rsgroup/src/main/java/org/apache/hadoop/hbase/rsgroup/RSGroupBasedLoadBalancer.java#L216

when fallback is enabled, servers does not contain the current group's rs, and contains the rs of other group, region will be assigend to other group, but assignments already contains targetRS, and then assignments.putAll overwrites old entry

this.internalBalancer.retainAssignment(currentAssignmentMap, candidateList)
              .forEach((serverName, regionInfos) -> {
                assignments.computeIfAbsent(serverName, s -> new ArrayList<>())
                    .addAll(regionInfos);
});

the issue exists only in the branch-2.