feat: Add .NET 8.0 support for Oracle persistence provider and improve test infrastructure

[henriquepagno]

Describe the change
This PR adds .NET 8.0 support to the Oracle persistence provider and improves the overall test infrastructure and documentation. The main highlight is the addition of .NET 8.0 compatibility, making the Oracle provider fully supported on the latest .NET version.

Describe your implementation or design

• Added .NET 8.0 support: Updated the Oracle persistence provider to target .NET 8.0 alongside existing .NET 6.0 support
• Improved test infrastructure:
  • Added Oracle tests to the GitHub CI/CD pipeline
  • Fixed Oracle tests using TestContainers for reliable containerized testing
  • Added .NET 8.0 test support for Oracle scenarios
• Enhanced documentation:
  • Improved Oracle persistence README with better usage examples and configuration details
  • Added Oracle as a persistence option to the main project README
• Code cleanup and maintenance:
  • Removed unnecessary SqlServer package dependencies from Oracle project
  • Cleaned up code files and made minor adjustments for better maintainability

Tests

• Oracle tests are now included in the GitHub pipeline and run successfully
• Tests use TestContainers for consistent and reliable Oracle database testing
• Added .NET 8.0 test support to ensure compatibility across both target frameworks
• All existing functionality remains intact and tested

Breaking change
No breaking changes. This is a backward-compatible enhancement that adds .NET 8.0 support while maintaining full compatibility with .NET 6.0.

Additional context
The Oracle persistence provider now fully supports the latest .NET 8.0 framework, making it ready for modern .NET applications. The improved test infrastructure ensures reliable testing across different environments, and the enhanced documentation makes it easier for developers to integrate Oracle persistence into their WorkflowCore applications.