Remove IAsyncEnumerable from DbSet

[roji]

Fixes #24041

Some compat notes:

• await foreach works fine, even if LangVersion is set to 7.0. Am not sure if this is because the SDK is new - if so, then new SDKs are required to build anything depending on EF Core 6.0 anyway (since we target net5.0).
• ToListAsync and friends work fine because they're over IQueryable

[Youssef1313]

await foreach works fine, even if LangVersion is set to 7.0. Am not sure if this is because the SDK is new - if so, then new SDKs are required to build anything depending on EF Core 6.0 anyway (since we target net5.0).

The SDK doesn't seem related I think. This is a compiler-specific thing. The compiler is expected to issue a compile-warning if await foreach is used but the type doesn't have GetAsyncEnumerator method (regardless whether it comes from the interface or not).

It's very odd that await foreach is still working. I can't think of a reason.

[roji]

The SDK doesn't seem related I think. This is a compiler-specific thing.

Yeah, I was referring to the compiler, which is delivered as part of the SDK.

Regardless, I agree this is odd, though unlikely to be important here specifically. Since EF Core 6.0 will target .NET 5.0 at a minimum, the default langversion is 9.0 in any case (and users will always have the option to use AsAsyncEnumerable if they need to).