Migrate BankSystem from .NET Core 3.1 to .NET 8

[mukesh-843]

This pull request migrates the BankSystem solution from .NET Core 3.1 (which is end-of-life) to .NET 8.

Changes

• Retargeted projects from netcoreapp3.1 to net8.0
• Updated NuGet package references for .NET 8 compatibility
• Replaced deprecated AesManaged with Aes.Create()
• Added explicit EF Core decimal precision configuration using HasPrecision(...)
• Replaced obsolete UseDatabaseErrorPage() with UseMigrationsEndPoint()
• Added AddDatabaseDeveloperPageExceptionFilter() for improved EF Core development diagnostics

Notes

• Environment-specific configuration and secrets were intentionally left unchanged.
• This PR focuses only on the .NET 8 migration and related compatibility updates.