Update Checkstyle DTD location

[DanVanAtta]

The sourceforge location is hidden, the updated location is
on https and recommended.

This issue goes into more details on the DTD migration:
checkstyle/checkstyle#6478

Testing

Screens Shots

Additional Notes to Reviewer

Release Note

[codecov]

Codecov Report

Merging #7579 into master will increase coverage by 0.21%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #7579      +/-   ##
============================================
+ Coverage     24.23%   24.45%   +0.21%     
- Complexity     7078     7160      +82     
============================================
  Files          1194     1212      +18     
  Lines         78953    79384     +431     
  Branches      11368    11393      +25     
============================================
+ Hits          19137    19414     +277     
- Misses        57623    57779     +156     
+ Partials       2193     2191       -2     

Impacted Files	Coverage Δ	Complexity Δ
...ric/xml/reader/exceptions/XmlParsingException.java	91.66% <0.00%> (-8.34%)	1.00% <0.00%> (ø%)
...java/org/triplea/generic/xml/reader/XmlMapper.java	88.46% <0.00%> (-1.34%)	17.00% <0.00%> (+2.00%)	⬇️
...n/java/org/triplea/game/server/AvailableGames.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...ames/strategy/engine/framework/ui/GameChooser.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...tegy/engine/data/gameparser/ShallowGameParser.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...y/engine/framework/ui/DefaultGameChooserEntry.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...n/java/org/triplea/map/data/elements/UnitList.java	100.00% <0.00%> (ø)	2.00% <0.00%> (?%)
...va/org/triplea/map/data/elements/ResourceList.java	100.00% <0.00%> (ø)	2.00% <0.00%> (?%)
...in/java/org/triplea/map/data/elements/Triplea.java	100.00% <0.00%> (ø)	2.00% <0.00%> (?%)
.../main/java/org/triplea/map/data/elements/Game.java	100.00% <0.00%> (ø)	17.00% <0.00%> (?%)
... and 21 more

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