3.x: Move Gradle properties into gradle.properties file

[benjamintboyle]

Moved version, group, and description into gradle.properties file.
Removed POM_ARTIFACT property as the 'name' property in the settings file
is used when building the POM. Removed 'release.scope' property as it
does not appear to be used. Removed VERSION_NAME property and references
to it, in favor of 'version' property.

[benjamintboyle]

Also verified that the POM created does not change. Generated POM before changes and diff'd it with POM after changes. The POMs are identical.

[codecov]

Codecov Report

Merging #7260 (b00718f) into 3.x (ac5e569) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7260      +/-   ##
============================================
- Coverage     99.56%   99.55%   -0.02%     
  Complexity     6757     6757              
============================================
  Files           747      747              
  Lines         47391    47391              
  Branches       6382     6382              
============================================
- Hits          47186    47180       -6     
- Misses           93       97       +4     
- Partials        112      114       +2     

Impacted Files	Coverage Δ	Complexity Δ
...nternal/operators/observable/ObservableCreate.java	96.58% <0.00%> (-3.42%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-3.18%)	2.00% <0.00%> (ø%)
...l/operators/observable/ObservableFlatMapMaybe.java	95.07% <0.00%> (-2.12%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.18% <0.00%> (-1.17%)	2.00% <0.00%> (ø%)
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	6.00% <0.00%> (ø%)
...3/internal/operators/flowable/FlowableGroupBy.java	85.02% <0.00%> (-0.30%)	3.00% <0.00%> (ø%)
.../operators/observable/ObservableCombineLatest.java	100.00% <0.00%> (+0.61%)	6.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableWindow.java	100.00% <0.00%> (+1.78%)	3.00% <0.00%> (ø%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	100.00% <0.00%> (+2.51%)	50.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	100.00% <0.00%> (+2.70%)	2.00% <0.00%> (ø%)

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[akarnokd]

Removing some of the properties might break the release plugin: https://github.com/vanniktech/gradle-maven-publish-plugin#setting-properties

[benjamintboyle]

Replaced property for release plugin.

[benjamintboyle]

VERSION_NAME and GROUP default to 'version' and 'group' as seen on Line 9 and 10 from Plugin source