3.x: Patch out duplicate @NonNull annotation in generated javadocs

[akarnokd]

There is a long-standing bug in either javac or javadocs that results in certain annotations to be put into the HTML twice:

https://bugs.openjdk.java.net/browse/JDK-8175533

Looks like it happens with certain @Documented annotations, but not others. The issue persists up to JDK 16 preview.

This PR adds a workaround to the existing HTML patching gradle task which removes such duplicates:

Sidenote: yes, the method return also duplicates @NonNull, but that's across HTML elements and is much harder to fix.

[codecov]

Codecov Report

Merging #7073 into 3.x will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7073      +/-   ##
============================================
+ Coverage     99.52%   99.54%   +0.02%     
  Complexity     6669     6669              
============================================
  Files           742      742              
  Lines         47270    47275       +5     
  Branches       6373     6374       +1     
============================================
+ Hits          47044    47059      +15     
  Misses          102      102              
+ Partials        124      114      -10     

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)	595.00 <0.00> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)	565.00 <0.00> (ø)
...a3/internal/jdk8/FlowableCollectWithCollector.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...ernal/jdk8/FlowableCollectWithCollectorSingle.java	100.00% <100.00%> (ø)	3.00 <1.00> (ø)
.../internal/jdk8/ObservableCollectWithCollector.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...nal/jdk8/ObservableCollectWithCollectorSingle.java	100.00% <100.00%> (ø)	3.00 <1.00> (ø)
...java3/internal/operators/flowable/FlowableMap.java	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...nternal/operators/observable/ObservableWindow.java	100.00% <100.00%> (+1.81%)	3.00 <0.00> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	91.74% <0.00%> (-6.43%)	2.00% <0.00%> (ø%)
.../internal/disposables/ListCompositeDisposable.java	98.00% <0.00%> (-2.00%)	34.00% <0.00%> (-1.00%)
... and 17 more

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