Upgrade license to GPL-3.0-or-later

[ssoloff]

Per #2764.

Things of note:

• I placed the license notice (with the exceptions) in the README. This seemed simpler than creating a separate file (e.g. LICENSE-NOTICE) that no one may actually read.
• I added a copyright notice to the license notice, as recommended in the GPL. I attributed the copyright to "TripleA contributors." Not sure if this is the best choice, but it really becomes moot if we adopt a CLA or a DCO (see Adopt a Contributor License Agreement #2880).
• The only exception I listed in the license notice was JavaMail. After further research, I concluded that it is probably unnecessary to list exceptions for libraries that we do not actually distribute with our release artifacts (which includes the JUnit 5 and JUnit 5 Samples libraries). If anyone disagrees, it's a simple matter to add the four additional libraries to the exceptions table.
• I believe the license badge in the README will automatically update once this PR is merged.

Unrelated to #2764:

• I changed the Markdown headings in the README to use # delimiters instead of underlining them with ===.
• I updated many links in the README to be canonical (e.g. use www.triplea-game.org instead of triplea-game.github.io), to use HTTPS if possible, etc.

[codecov-io]

Codecov Report

Merging #2949 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2949      +/-   ##
============================================
- Coverage      20.4%   20.39%   -0.01%     
+ Complexity     5770     5769       -1     
============================================
  Files           817      817              
  Lines         72905    72906       +1     
  Branches      12035    12035              
============================================
- Hits          14876    14872       -4     
- Misses        55933    55936       +3     
- Partials       2096     2098       +2

Impacted Files	Coverage Δ	Complexity Δ
...va/games/strategy/triplea/ui/menubar/HelpMenu.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...main/java/games/strategy/triplea/UrlConstants.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...rategy/triplea/attachments/UnitTypeComparator.java	35.71% <0%> (-7.15%)	10% <0%> (-1%)
...rc/main/java/games/strategy/net/nio/NioWriter.java	66.94% <0%> (-1.7%)	20% <0%> (ø)
...tegy/triplea/oddsCalculator/ta/OddsCalculator.java	43.39% <0%> (-0.32%)	13% <0%> (ø)

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[RoiEXLab]

www.triplea-game.org should be just triplea-game.org

[ssoloff]

442d8fa

[RoiEXLab]

I believe for this PR @veqryn @ron-murhammer and @DanVanAtta should all approve...

[RoiEXLab]

Unrelated, but the Ice Box label was renamed to ice box - close and revisit later, so this link is broken.
Additionally this link should be hidden behind a nicer name, it really does not look good in the mark down.

[ssoloff]

Agreed. In general, I don't like dropping bare URLs in Markdown, especially ones as busy as the ice box link. How about if I just hide all links in the README behind a label? For example:

Development

• Project documentation (including 'how to get started guides')
• Broken map list
• Additional feature request list (list is not to be added to, if picked up, re-open and remove the ice-box label)

[RoiEXLab]

👍 For consistency

[ssoloff]

0752369

Thanks for catching the broken ice box link. I tested all the others; not sure why I skipped that one. 🤷‍

[ssoloff]

I believe the license badge in the README will automatically update once this PR is merged.

Confirmed.