Improve support for JDK 11 builds

[uhafner]

Rework of some additional elements to use JDK 11 as build environment:

• Enable animal sniffer for JDK 8
• Create a Jenkins agent for Java 11
• Fix some warnings due to Error Prone upgrade

[codecov-io]

Codecov Report

Merging #299 (692de91) into master (df3c8a0) will decrease coverage by 0.58%.
The diff coverage is 80.95%.

@@             Coverage Diff              @@
##             master     #299      +/-   ##
============================================
- Coverage     88.19%   87.61%   -0.59%     
  Complexity       79       79              
============================================
  Files             8        8              
  Lines           322      323       +1     
  Branches         47       48       +1     
============================================
- Hits            284      283       -1     
- Misses           30       31       +1     
- Partials          8        9       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/edu/hm/hafner/util/Ensure.java	75.20% <78.94%> (-1.42%)	8.00 <0.00> (ø)
src/main/java/edu/hm/hafner/util/FilteredLog.java	100.00% <100.00%> (ø)	15.00 <0.00> (ø)
...ain/java/edu/hm/hafner/util/TreeStringBuilder.java	94.33% <100.00%> (ø)	6.00 <0.00> (ø)

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