Bump .NET SDK version to 8

[frogcrush]

PR Type

What kind of change does this PR introduce?

• Feature

What is the current behavior?

.NET SDK targets version 7

What is the new behavior?

Uses the official .NET 8 SDK image

PR Checklist

Please check if your PR fulfills the following requirements:

• [ X ] Tested code with current supported SDKs
• [ ] Docs have been added/updated which fit documentation template. (for bug fixes / features)
• [ ] Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• [ ] Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• [ ] Contains NO breaking changes
• [ ] Updated the Release Notes
• [ ] Associated with an issue (GitHub or internal)

Other information

Tested locally. Information in https://github.com/unoplatform/docker/issues/15 seems to be incorrect as the build will fail without all of the dependencies included in the wasm-build image.