Fix findbugs warnings in hadoop-yarn-server-resourcemanager module

Recently there are 2 extant Findbugs warnings in hadoop-yarn-server-resourcemanager module according to the jenkins report of YARN-9439/YARN-9440 as follow:

They seems not expect null input for com.google.common.util.concurrent.SettableFuture#set even through they are indeed declared as Nullable, perhaps since findbugs can support javax.annotation.Nullable but not org.checkerframework.checker.nullness.qual.Nullable.
I think we should exclude these two warnings through adding them into hadoop-yarn/dev-support/findbugs-exclude.xml.