[Feature Contribution]: Upgrade .NET version to .NET 8

[tiwatsuka]

What happened?

.NET 8 that is the next LTS version of .NET is planned to be release Nov. 2023. Carbon Aware SDK should be upgraded to .NET 8 because using the latest LTS is good practice, generally.

FYI, .NET 8 contains important improvements to implement #400.

• https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-8#metrics
• https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-8#imeterfactory-interface

Code of Conduct

• I agree to follow this project's Code of Conduct

Feature Commitment

• I commit to contributing this feature as a PR and working with the GSF to merge this feature into the Carbon Aware SDK.

[Sophietn]

Blocked until #420 is done