Diamond operators in unit tests, observable package

[dvolkovv]

Hello, in this pull request i've changed all IDE marked explicit types with diamond operator. Affected package is internal/operators/observable. There is one test fail in CompletableTest.repeatNormal , but diamond is not the cause, there is last stack entry:
java.lang.AssertionError: expected:<6> but was:<5>
at org.junit.Assert.fail(Assert.java:88)

This PR is part of #6767 issue resolving.

[codecov]

Codecov Report

Merging #6786 into 3.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6786      +/-   ##
============================================
+ Coverage     98.24%   98.27%   +0.03%     
- Complexity     6286     6288       +2     
============================================
  Files           696      696              
  Lines         45390    45390              
  Branches       6233     6233              
============================================
+ Hits          44592    44609      +17     
+ Misses          277      268       -9     
+ Partials        521      513       -8

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...va3/internal/operators/parallel/ParallelRunOn.java	96.61% <0%> (-0.49%)	8% <0%> (ø)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	97.09% <0%> (-0.42%)	50% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	89.63% <0%> (-0.34%)	3% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (ø)	2% <0%> (ø)	⬇️
...3/internal/operators/flowable/FlowableGroupBy.java	95.91% <0%> (+0.31%)	3% <0%> (ø)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	97.73% <0%> (+0.32%)	6% <0%> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	95.83% <0%> (+0.52%)	2% <0%> (ø)	⬇️
...rnal/operators/observable/ObservableSwitchMap.java	93.68% <0%> (+0.52%)	3% <0%> (ø)	⬇️
... and 8 more

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