Improved PR template

[ham1]

Description

Removed checkboxes from items which don't all have to be checked.
Added link to code style guidelines (the wiki page should be further updated).

[codecov-io]

Codecov Report

Merging #326 into trunk will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##              trunk     #326      +/-   ##
============================================
+ Coverage     57.74%   57.75%   +<.01%     
- Complexity     9913     9914       +1     
============================================
  Files          1139     1139              
  Lines         73051    73051              
  Branches       7303     7303              
============================================
+ Hits          42185    42190       +5     
+ Misses        28393    28389       -4     
+ Partials       2473     2472       -1

Impacted Files	Coverage Δ	Complexity Δ
...c/core/org/apache/jmeter/reporters/Summariser.java	84.61% <0%> (-0.77%)	17% <0%> (-1%)
...apache/jmeter/extractor/TestBoundaryExtractor.java	97.88% <0%> (ø)	17% <0%> (ø)	⬇️
...mpler/hc/JMeterPoolingClientConnectionManager.java	30.95% <0%> (+3.17%)	11% <0%> (+1%)	⬆️
...s/org/apache/jmeter/timers/PoissonRandomTimer.java	78.37% <0%> (+5.4%)	10% <0%> (+1%)	⬆️

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[ham1]

How can this PR affect code coverage? Do we have tests which are non-deterministic and then change the test coverage each run?