Modernize gradle plugin block, change maven to maven-publish

[benjamintboyle]

Move gradle plugins to block at top of file per gradle recommendations.
Change deprecated maven plugin to maven-publish as in Gradle 7.0 it will
be removed. Several other minor updates to gradle file including, removing
semi-colons, changing println to use built-in logger.

[codecov]

Codecov Report

Merging #7219 (c0301bc) into 3.x (c38ea99) will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7219      +/-   ##
============================================
+ Coverage     99.50%   99.56%   +0.05%     
+ Complexity     6757     6756       -1     
============================================
  Files           747      747              
  Lines         47391    47391              
  Branches       6382     6382              
============================================
+ Hits          47157    47183      +26     
+ Misses          105       93      -12     
+ Partials        129      115      -14     

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-2.39%)	2.00% <0.00%> (ø%)
...va/io/reactivex/rxjava3/subjects/AsyncSubject.java	97.87% <0.00%> (-2.13%)	44.00% <0.00%> (-1.00%)
...lowable/FlowableFlatMapCompletableCompletable.java	98.68% <0.00%> (-1.32%)	3.00% <0.00%> (ø%)
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	6.00% <0.00%> (ø%)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	99.18% <0.00%> (-0.82%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	6.00% <0.00%> (ø%)
.../reactivex/rxjava3/observers/BaseTestConsumer.java	99.46% <0.00%> (-0.54%)	74.00% <0.00%> (-1.00%)
...a3/internal/operators/flowable/FlowableCreate.java	99.02% <0.00%> (-0.33%)	6.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableFlatMap.java	98.22% <0.00%> (+0.35%)	3.00% <0.00%> (ø%)
...operators/flowable/FlowableConcatMapScheduler.java	99.60% <0.00%> (+0.39%)	4.00% <0.00%> (ø%)
... and 8 more

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