Fix up handling for non-generic Task and ValueTask

[captainsafia]

Closes #61019.

[BrennanConroy]

Was trying to understand what exactly this did, and why we couldn't just get rid of this check since ReplaceGenericArguments below should handle Task<T>.
Looks like ConstructedFrom would return Task<int> if you had a type like Example : Task<int>. Is that the desired effect? We should probably add a test like that.

We also have tests for Task and Task<T>

aspnetcore/src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/snapshots/CompletenessTests.SupportsAllXmlTagsOnSchemas#OpenApiXmlCommentSupport.generated.verified.cs

Lines 274 to 276 in 394df7a

	_cache.Add(new MemberKey(typeof(global::ExampleClass), MemberType.Method, "AddAsync", typeof(global::System.Threading.Tasks.Task<>), [typeof(global::System.Int32), typeof(global::System.Int32)]), new XmlComment(@"This method is an example of a method that
	returns an awaitable item.", null, null, null, null, false, null, null, null));
	_cache.Add(new MemberKey(typeof(global::ExampleClass), MemberType.Method, "DoNothingAsync", typeof(global::System.Threading.Tasks.Task), []), new XmlComment(@"This method is an example of a method that

So not too sure what was broken.

[captainsafia]

Closing in favor of #61145.