Update Pull Request template

[bjoernricks]

What:

Try to improve the quality of information being put into pull requests.

Why:

Often the PR only contains the what part. Sometimes also the why part.
But never the how part.

How:

Sadly the new PR template can only be tested after the PR got merged.

Checklist:

• Tests (N/A)
• CHANGELOG Entry (N/A)

[codecov]

Codecov Report

Merging #2417 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2417      +/-   ##
==========================================
- Coverage   54.96%   54.89%   -0.07%     
==========================================
  Files        1094     1093       -1     
  Lines       26573    26603      +30     
  Branches     7537     7570      +33     
==========================================
- Hits        14605    14603       -2     
- Misses      10862    10895      +33     
+ Partials     1106     1105       -1     

Impacted Files	Coverage Δ
gsa/src/web/pages/alerts/component.js	19.08% <0.00%> (-3.14%)	⬇️
gsa/src/web/pages/alerts/dialog.js	20.33% <0.00%> (-0.85%)	⬇️
gsa/src/web/utils/stateReducer.js

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