Treat all source files as UTF-8 for java, javadoc, gatkdoc.

[cmnbroad]

I tested this using the following sketchy procedure: I temporarily reverted #5936 on this branch, thereby re-introducing non-ASCII characters into the source. That builds without error, as it should. Then I temporarily changed the newly added encoding declarations included this PR in build.gradle from "UTF-8" to "US-ASCII", after which I was able to reproduce exactly the same errors as reported in #5934, for both compile and gatkDoc tasks. So I think these changes achieve the desired result (accept UTF-8 in source).

[codecov]

Codecov Report

Merging #5946 into master will increase coverage by 0.005%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##              master     #5946       +/-   ##
===============================================
+ Coverage     86.817%   86.823%   +0.005%     
  Complexity     32344     32344               
===============================================
  Files           1993      1993               
  Lines         149460    149460               
  Branches       16502     16502               
===============================================
+ Hits          129757    129765        +8     
+ Misses         13687     13679        -8     
  Partials        6016      6016

Impacted Files	Coverage Δ	Complexity Δ
...hellbender/utils/pairhmm/VectorLoglessPairHMM.java	86.842% <0%> (+1.316%)	12% <0%> (ø)	⬇️
.../hellbender/utils/python/PythonScriptExecutor.java	66.667% <0%> (+3.333%)	10% <0%> (ø)	⬇️
...oadinstitute/hellbender/utils/pairhmm/PairHMM.java	78.417% <0%> (+3.597%)	24% <0%> (ø)	⬇️

[lbergelson]

👍 Thank you @cmnbroad.