Replace the content of LICENSE.txt file by the GPL 3 license text

[sequba]

Context

@krzysztofspilka wrote in #1015 :

For commercial use, our proprietary license replaces the open-source terms which gives us an opportunity to better communicate the open-source character of HyperFormula. We could do that by replacing the content of the LICENSE.txt file with the content already existing in gpl-3.0.txt.

Both GitHub and npm should properly detect, and display the new license.

The file "gpl-3.0.txt" should be then deleted.

How has this been tested?

Local docs build

Types of changes

• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] New feature or improvement (non-breaking change which adds functionality)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Additional language file or change to the existing one (translations)
• [x] Change to the documentation

Related issues:

Fixes #1015

Checklist:

• [x] My code follows the code style of this project.
• [ ] I described the modification in the CHANGELOG.md file.
• [x] My change requires a change to the documentation.
• [ ] My change requires a migration guide.

[codecov[bot]]

Codecov Report

Merging #1044 (0d8bc04) into develop (52b284f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1044   +/-   ##
========================================
  Coverage    95.74%   95.74%           
========================================
  Files          161      161           
  Lines        14085    14085           
  Branches      2920     2974   +54     
========================================
  Hits         13485    13485           
+ Misses         600      584   -16     
- Partials         0       16   +16     

Impacted Files	Coverage Δ
src/ArrayValue.ts	81.17% <0.00%> (ø)
src/HyperFormula.ts	92.73% <0.00%> (ø)
src/DependencyGraph/DependencyGraph.ts	97.94% <0.00%> (ø)
src/DependencyGraph/FormulaCellVertex.ts	84.00% <0.00%> (ø)
src/interpreter/plugin/3rdparty/bessel/bessel.ts	83.75% <0.00%> (ø)
...rc/DependencyGraph/AddressMapping/DenseStrategy.ts	64.58% <0.00%> (ø)
...c/DependencyGraph/AddressMapping/SparseStrategy.ts	85.58% <0.00%> (ø)