Use `ISession.FlushAsync()` instead of `SaveChangesAsync()` (OSOE-1036)

[Piedone]

Here:

Helpful-Libraries/Lombiq.HelpfulLibraries.Tests/UnitTests/Services/ManualConnectingIndexServiceFixture.cs

Line 49 in 5f1febb

await session.SaveChangesAsync();

For the reason see OrchardCMS/OrchardCore#17084 (comment).

Check if the feature still works and it indeed doesn't need SaveChangesAsync(), even in the ReloadShellAfterMaintenanceCompletion case.

Related: Lombiq/Hosting-Tenants#149.

Jira issue