Bump gradle to 6.8.3 & optimize gradle config

[Goooler]

• Update gradle 6.8.3
• Replace https://plugins.gradle.org/m2/ with gradlePluginPortal()
• Put versions into ext block

[codecov]

Codecov Report

Merging #7207 (4d585bb) into 3.x (f75498c) will increase coverage by 0.10%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7207      +/-   ##
============================================
+ Coverage     99.47%   99.58%   +0.10%     
  Complexity     6758     6758              
============================================
  Files           747      747              
  Lines         47392    47392              
  Branches       6382     6382              
============================================
+ Hits          47145    47194      +49     
+ Misses          112       91      -21     
+ Partials        135      107      -28     

Impacted Files	Coverage Δ	Complexity Δ
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	10.00% <0.00%> (-1.00%)
...operators/flowable/FlowableConcatMapScheduler.java	99.60% <0.00%> (+0.39%)	4.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableReplay.java	100.00% <0.00%> (+0.53%)	22.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableFlatMap.java	98.22% <0.00%> (+0.70%)	3.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableCreate.java	99.14% <0.00%> (+0.85%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableSequenceEqual.java	100.00% <0.00%> (+1.21%)	2.00% <0.00%> (ø%)
...operators/flowable/FlowableFlatMapCompletable.java	100.00% <0.00%> (+1.28%)	2.00% <0.00%> (ø%)
...rxjava3/internal/schedulers/ExecutorScheduler.java	100.00% <0.00%> (+1.31%)	10.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	98.64% <0.00%> (+1.35%)	2.00% <0.00%> (ø%)
...internal/operators/flowable/FlowableSwitchMap.java	100.00% <0.00%> (+1.41%)	3.00% <0.00%> (ø%)
... and 8 more

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[akarnokd]

reopening