Complete the switch from Travis-CI to GitHub Actions

[x] GitHub Actions should run on every push and on every pull request.
[x] Deploy to MyGet on every successful build on the develop branch.
[x] Deploy to NuGet on every successful build on the master branch.
[x] There needs to be a PropertyGroup for GitHub Actions like we have for other CI providers.
[x] NtsOfficialRelease should be true when building the master branch on GitHub Actions.
[x] NtsOfficialRelease should not be true in any other case.
[x] Once everything else is done, disable Travis-CI on the backend and delete .travis.yml

NetTopologySuite / NetTopologySuite