3.x: Fix type inference problem when compiling with newer JDKs

[akarnokd]

Code in the FlowableGroupByTest didn't compile on newer JDKs because of type inference changes between Java 6 and them. In Java 6, this

Flowable<Integer> source = ...
Flowable<X> result = source.groupBy(Functions.identity());

comes out as X == GroupedFlowable<Object, Integer>. However, newer JDKs will propagate type arguments thus:

Flowable<Integer> source = ...
Flowable<Y> result = source.groupBy(Functions.identity());

Y == GroupedFlowable<Integer, Integer> and mismatching with the rest of the types.

I chose an explicit Integer argument for clarity.

[codecov]

Codecov Report

Merging #6682 into 3.x will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6682      +/-   ##
============================================
+ Coverage     98.12%   98.14%   +0.02%     
- Complexity     6187     6188       +1     
============================================
  Files           677      677              
  Lines         44657    44657              
  Branches       6170     6170              
============================================
+ Hits          43819    43828       +9     
+ Misses          300      294       -6     
+ Partials        538      535       -3

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.22% <0%> (-3.13%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	92.25% <0%> (-2.12%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	94.39% <0%> (-1.41%)	3% <0%> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0%> (-0.91%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	96.58% <0%> (-0.86%)	2% <0%> (ø)
...va3/internal/operators/flowable/FlowableCache.java	97.72% <0%> (-0.76%)	37% <0%> (-1%)
...3/internal/operators/flowable/FlowableGroupBy.java	95.75% <0%> (-0.54%)	3% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	97.08% <0%> (-0.33%)	6% <0%> (ø)
... and 13 more

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