3.x: Migrate Gradle to Java Toolchain

[benjamintboyle]

Use Java 8 as toolchain for Java tasks, instead of using JAVA_HOME and
removing need to cross-compile.

[codecov]

Codecov Report

Merging #7239 (40df0e3) into 3.x (5bb119b) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7239      +/-   ##
============================================
- Coverage     99.55%   99.54%   -0.02%     
  Complexity     6757     6757              
============================================
  Files           747      747              
  Lines         47391    47391              
  Branches       6382     6382              
============================================
- Hits          47181    47174       -7     
- Misses           96       98       +2     
- Partials        114      119       +5     

Impacted Files	Coverage Δ	Complexity Δ
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	10.00% <0.00%> (-1.00%)
...l/operators/observable/ObservableFlatMapMaybe.java	91.54% <0.00%> (-3.53%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-1.59%)	2.00% <0.00%> (ø%)
...3/internal/operators/flowable/FlowablePublish.java	99.00% <0.00%> (-1.00%)	17.00% <0.00%> (-1.00%)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	99.16% <0.00%> (-0.42%)	49.00% <0.00%> (ø%)
...operators/flowable/FlowableConcatMapScheduler.java	99.20% <0.00%> (-0.40%)	4.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableCreate.java	99.02% <0.00%> (-0.33%)	6.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableFlatMap.java	97.87% <0.00%> (+0.35%)	3.00% <0.00%> (ø%)
.../operators/observable/ObservableCombineLatest.java	100.00% <0.00%> (+0.61%)	6.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	97.92% <0.00%> (+1.03%)	2.00% <0.00%> (ø%)
... and 2 more

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