📝 improve dotnet release versions section

[Odonno]

• Change rules about .NET release version (LTS, STS, .NET Standard 2.1)
• Display the list of currently supported .NET versions, with their official End of Support
• Add .NET 7 as a target framework for the library

Note that I did not see any changes needed to be done for .NET 7, nor did I see any noticeable performance improvement.

[s-e-f]

Note that benchmarks and tests need to be updated to target this additional runtime version too.

[s-e-f]

Using Directory.Build.props can help in centralizing and syncing the different runtime versions across projects.

[s-e-f]

For the readme, perhaps we can simplify the support structure to something along the lines of:

SurrealDb.Net targets .NET versions that are in support by Microsoft.

Additionally, SurrealDb.Net targets .NET Standard 2.1 explicitly for the Unity ecosystem.

A link to the Microsoft .NET support window might be helpful too to clarify the difference between STS/LTS, exact dates from the source. This also covers the overlap that exists for a short period between .NET releases, which is technically not covered by the current explanation.

[Odonno]

Note that benchmarks and tests need to be updated to target this additional runtime version too.

I tested these projects locally with .NET 7, but yes, I commit & push the upgrade.

[Odonno]

Using Directory.Build.props can help in centralizing and syncing the different runtime versions across projects.

Using this file is planned to centralize used packages, versioning, etc... I think there is not enough projects in the solution to implement it at the moment but it can be a nice addition later.