Update most recent Java version and Gradle version

[lorenzleutgeb]

Java 13 went out of "Premier Support" in March, therefore I removed it. Also, there is now Java 14, which is not yet succeeded by a more recent release, so I added it.

[codecov]

Codecov Report

Merging #257 into master will increase coverage by 2.18%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #257      +/-   ##
============================================
+ Coverage     77.30%   79.49%   +2.18%     
- Complexity     2108     2783     +675     
============================================
  Files           157      186      +29     
  Lines          6884     9242    +2358     
  Branches       1181     1726     +545     
============================================
+ Hits           5322     7347    +2025     
- Misses         1167     1419     +252     
- Partials        395      476      +81     

Impacted Files	Coverage Δ	Complexity Δ
.../ac/tuwien/kr/alpha/grounder/atoms/ChoiceAtom.java	50.98% <0.00%> (-36.52%)	16.00% <0.00%> (+7.00%)	⬇️
...java/at/ac/tuwien/kr/alpha/config/InputConfig.java	63.33% <0.00%> (-11.09%)	26.00% <0.00%> (+9.00%)	⬇️
.../ac/tuwien/kr/alpha/common/atoms/ExternalAtom.java	71.42% <0.00%> (-5.24%)	28.00% <0.00%> (+16.00%)	⬇️
...at/ac/tuwien/kr/alpha/grounder/atoms/RuleAtom.java	67.56% <0.00%> (-4.44%)	16.00% <0.00%> (+6.00%)	⬇️
...c/tuwien/kr/alpha/grounder/RuleGroundingOrder.java	86.66% <0.00%> (-3.96%)	12.00% <0.00%> (+1.00%)	⬇️
...t/ac/tuwien/kr/alpha/grounder/BridgedGrounder.java	26.66% <0.00%> (-3.34%)	1.00% <0.00%> (ø%)
src/main/java/at/ac/tuwien/kr/alpha/Main.java	45.80% <0.00%> (-0.98%)	9.00% <0.00%> (+4.00%)	⬇️
...t/ac/tuwien/kr/alpha/config/CommandLineParser.java	81.60% <0.00%> (-0.57%)	43.00% <0.00%> (+17.00%)	⬇️
...uwien/kr/alpha/common/depgraph/ComponentGraph.java	98.41% <0.00%> (ø)	4.00% <0.00%> (?%)
...pha/common/depgraph/DepthFirstSearchAlgorithm.java	100.00% <0.00%> (ø)	10.00% <0.00%> (?%)
... and 62 more

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[lorenzleutgeb]

Changes to gradle/* and gradlew* were automatically generated by ./gradlew wrapper.