3.x: Add step to JDK 11 build to verify generated module-info

[akarnokd]

When building the module-info via the beryx plugin on Java 8, there is no verification for the contents possible. Therefore, the plugin documentation suggest building with a newer JDK and specifying a 9+ target.

Related #7241

[codecov]

Codecov Report

Merging #7334 (74143a9) into 3.x (ce45fc2) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7334      +/-   ##
============================================
- Coverage     99.55%   99.53%   -0.03%     
+ Complexity     6781     6779       -2     
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47271    47260      -11     
- Misses           94      100       +6     
- Partials        117      122       +5     

Impacted Files	Coverage Δ
...vex/rxjava3/internal/operators/maybe/MaybeAmb.java	96.96% <0.00%> (-3.04%)	⬇️
...va/io/reactivex/rxjava3/subjects/MaybeSubject.java	97.75% <0.00%> (-2.25%)	⬇️
...va/io/reactivex/rxjava3/subjects/AsyncSubject.java	97.87% <0.00%> (-2.13%)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0.00%> (-1.99%)	⬇️
...nternal/operators/observable/ObservableCreate.java	96.58% <0.00%> (-1.71%)	⬇️
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	94.36% <0.00%> (-0.71%)	⬇️
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	⬇️
...operators/flowable/FlowableConcatMapScheduler.java	99.20% <0.00%> (-0.40%)	⬇️
...ternal/operators/observable/ObservableFlatMap.java	97.87% <0.00%> (-0.36%)	⬇️
... and 5 more

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