Add .NET 8.0 support to MySQL persistence provider to fix CoreTypeMappingParameters error

[Copilot]

This PR fixes the .NET 8.0 MySQL compatibility issue where applications would encounter a CoreTypeMappingParameters constructor error when using the MySQL persistence provider.

Problem

When using WorkflowCore with MySQL persistence in .NET 8.0 applications, users encountered this runtime error:

Method not found: 'Void CoreTypeMappingParameters..ctor(System.Type, Microsoft.EntityFrameworkCore.Storage.ValueConversion.ValueConverter, Microsoft.EntityFrameworkCore.ChangeTracking.ValueComparer, Microsoft.EntityFrameworkCore.ChangeTracking.ValueComparer, Microsoft.EntityFrameworkCore.ChangeTracking.ValueComparer, System.Func`3<Microsoft.EntityFrameworkCore.Metadata.IProperty,Microsoft.EntityFrameworkCore.Metadata.IEntityType,Microsoft.EntityFrameworkCore.ValueGeneration.ValueGenerator>)'

at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlBoolTypeMapping..ctor(String storeType, Nullable`1 dbType, Nullable`1 size)

The issue occurred because the MySQL persistence provider only targeted netstandard2.1 and net6.0, causing .NET 8.0 applications to use incompatible versions of Pomelo.EntityFrameworkCore.MySql.

Solution

• Added net8.0 target framework to WorkflowCore.Persistence.MySQL.csproj
• Added .NET 8.0 specific package references:
  • Pomelo.EntityFrameworkCore.MySql version 8.*
  • Microsoft.EntityFrameworkCore.Tools version 8.*
• Updated the test project to include net8.0 target framework

This approach follows the same pattern used by other persistence providers (SQL Server and PostgreSQL) which already have .NET 8.0 support.

Testing

• All 21 MySQL tests pass successfully on .NET 8.0
• Backward compatibility maintained for existing target frameworks (netstandard2.1, net6.0)
• Verified that the CoreTypeMappingParameters error no longer occurs in .NET 8.0 applications

Fixes #1378.

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