Bump NSubstitute from 5.1.0 to 6.0.0 - #16

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-dotnet@v4, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

'PostgreSqlBuilder.PostgreSqlBuilder()' is obsolete: 'This parameterless constructor is obsolete and will be removed. Use the constructor with the image parameter instead: https://github.com/testcontainers/testcontainers-dotnet/discussions/1470#discussioncomment-15185721.'

'PostgreSqlBuilder.PostgreSqlBuilder()' is obsolete: 'This parameterless constructor is obsolete and will be removed. Use the constructor with the image parameter instead: https://github.com/testcontainers/testcontainers-dotnet/discussions/1470#discussioncomment-15185721.'

'PostgreSqlBuilder.PostgreSqlBuilder()' is obsolete: 'This parameterless constructor is obsolete and will be removed. Use the constructor with the image parameter instead: https://github.com/testcontainers/testcontainers-dotnet/discussions/1470#discussioncomment-15185721.'

'PostgreSqlBuilder.PostgreSqlBuilder()' is obsolete: 'This parameterless constructor is obsolete and will be removed. Use the constructor with the image parameter instead: https://github.com/testcontainers/testcontainers-dotnet/discussions/1470#discussioncomment-15185721.'

'PostgreSqlBuilder.PostgreSqlBuilder()' is obsolete: 'This parameterless constructor is obsolete and will be removed. Use the constructor with the image parameter instead: https://github.com/testcontainers/testcontainers-dotnet/discussions/1470#discussioncomment-15185721.'