Update to .NET 8.0

[imnasnainaec]

As an organization (SIL > ILS > LangTech > LexTools), we're wanting to push everything forward to .NET 8.0 where possible, for future-looking, cross-platform maintenance and support.

[imnasnainaec]

Changes:

• https://learn.microsoft.com/en-us/aspnet/core/migration/60-70
• https://learn.microsoft.com/en-us/aspnet/core/migration/70-80
• https://learn.microsoft.com/en-us/dotnet/core/compatibility/7.0
• https://learn.microsoft.com/en-us/dotnet/core/compatibility/8.0
• https://learn.microsoft.com/en-us/aspnet/core/release-notes/aspnetcore-7.0
• https://learn.microsoft.com/en-us/aspnet/core/release-notes/aspnetcore-8.0

[imnasnainaec]

Files to update:

• https://github.com/sillsdev/TheCombine/blob/master/README.md?plain=1#L104
• https://github.com/sillsdev/TheCombine/blob/master/Backend/BackendFramework.csproj#L3-L19
• https://github.com/sillsdev/TheCombine/blob/master/Backend.Tests/Backend.Tests.csproj#L3
• https://github.com/sillsdev/TheCombine/blob/master/Backend/Dockerfile#L2-L14
• https://github.com/sillsdev/TheCombine/blob/master/.github/workflows/backend.yml#L17-L130
• https://github.com/sillsdev/TheCombine/blob/master/.vscode/launch.json#L27
• Update backend licenses
• Add .env.development with https://learn.microsoft.com/en-us/aspnet/core/release-notes/aspnetcore-7.0#configure-dotnet-watch-to-always-restart-for-rude-edits

And watch out for:

• https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/8.0/file-path-backslash
• https://learn.microsoft.com/en-us/dotnet/core/compatibility/aspnet-core/8.0/antiforgery-checks