Update DatabaseMigrations and AspireShop to EF 9

[AndriySvyryd]

Related to dotnet/aspire#5207

• EnsureDatabaseAsync should be removed as now this is handled well by MigrateAsync
• MigrateAsync shouldn't be wrapped in a transaction and execution strategy. This is now done internally, and the explicit wrapping can be detrimental in some cases (for migration operations that can't be executed in a transaction).
• Seeding should be done using the new UseAsyncSeeding method on DbContextOptionsBuilder. See Upgrade EF to 9.0 eShop#488 for additional suggestions.

Also, use WaitForResourceAsync(...) when available.

[DamianEdwards]

We're not planning on updating the samples in this repo to .NET 9 but rather keeping them targeting .NET 8 but using Aspire 9, as .NET 8 is LTS. Using EF Core 9 lifts dependencies to .NET 9 versions thus pulling the project out of LTS so we'll keep them on EF Core 8 too. I'll leave this open though to track doing this work when we move the samples to .NET 10.

[herzkoenig]

It would be great if you could share the recommended final changes here in this issue if you’re not planning to update the samples.