project type guid and tests

[dzsquared]

Description

fixes #740 with the project type guid we added to slngen (https://github.com/microsoft/slngen/pull/623/files) and documented (https://learn.microsoft.com/en-us/sql/tools/sql-database-projects/howto/convert-original-sql-project?view=sql-server-ver17&pivots=sq1-visual-studio-code)

follows the logic expected by the .NET SDK - https://github.com/search?q=repo%3Adotnet%2Fsdk+DefaultProjectTypeGuid&type=code

ran into some mono issues with .NET Framework locally and the sln test, so both tests are .NET 8+ only

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why it's being changed, and any links to relevant issues. If this is closing an existing issue use one of the issue linking keywords to link the issue to this PR and have it automatically close when completed.

In addition, go through the checklist below and check each item as you validate it is either handled or not applicable to this change.

Code Changes

• Unit tests are added, if possible
• Existing tests are passing
• New or updated code follows the guidelines here
• Ensure .dacpac from changes to Microsoft.Build.Sql build process MUST be backwards compatible with older versions of SqlPackage
• Use proper logging for MSBuild tasks