3.x: Adjust infrastructure version markers

[akarnokd]

This PR updates the 2.x and rxjava2 indicators in some documents and build scripts to target RxJava 3 to io.reactivex.rxjava3.

[vanniktech]

I'm not sure how I feel about a possible version 3

[codecov]

Codecov Report

Merging #6421 into 3.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6421      +/-   ##
============================================
- Coverage     98.34%   98.28%   -0.07%     
+ Complexity     6300     6298       -2     
============================================
  Files           675      675              
  Lines         45172    45172              
  Branches       6246     6246              
============================================
- Hits          44425    44396      -29     
- Misses          229      238       +9     
- Partials        518      538      +20

Impacted Files	Coverage Δ	Complexity Δ
...tivex/internal/operators/single/SingleTimeout.java	96.66% <0%> (-3.34%)	2% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	93.06% <0%> (-1.99%)	2% <0%> (ø)
...internal/operators/observable/ObservableCache.java	95.9% <0%> (-1.64%)	34% <0%> (-1%)
...perators/mixed/ObservableConcatMapCompletable.java	98.49% <0%> (-1.51%)	3% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	94.16% <0%> (-1.46%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.42%)	3% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...nal/operators/observable/ObservableScalarXMap.java	96.55% <0%> (-1.15%)	7% <0%> (ø)
... and 24 more

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[anand-prakash-1]

HI, this is an .xml file containing some bugs found on this project using FindBugs. Just wanted to report them to you guys. Hope it's useful :)

RxJava-2.x.fbwarnings.zip