Enable nullability and fix warnings

[gewarren]

Enables nullability in 50 C# projects.
Also removes a few BinaryFormatter examples.

Fixes #8986.

[BillWagner]

This is great work @gewarren

I had a few optional suggestions. You should decide which style best illustrates the sample, then let's

[learn-build-service-prod]

Learn Build status updates of commit c93d5e8:

✅ Validation status: passed

File	Status	Preview URL	Details
snippets/csharp/Microsoft.Extensions.DependencyModel/DependencyContext/Overview/DependencyContextSnippets.csproj	✅Succeeded
snippets/csharp/Microsoft.SqlServer.Server/SqlFunctionAttribute/Function.csproj	✅Succeeded
snippets/csharp/Microsoft.SqlServer.Server/SqlUserDefinedAggregateAttribute/Aggregate.csproj	✅Succeeded
snippets/csharp/Microsoft.SqlServer.Server/SqlUserDefinedTypeAttribute/Type.csproj	✅Succeeded
snippets/csharp/Microsoft.SqlServer.Server/SqlUserDefinedTypeAttribute/Type1.cs	✅Succeeded
snippets/csharp/System.Collections.Immutable/ImmutableArray`1/Overview/ImmutableArraySnippets.csproj	✅Succeeded
snippets/csharp/System.DateTime/DateWithTimeZone.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System.DateTime/Persistence.cs	✅Succeeded	View
snippets/csharp/System.DateTime/SystemDateTimeReference.csproj	✅Succeeded
snippets/csharp/System.Diagnostics.Tracing/EventSource/Overview/etwtracesmall.csproj	✅Succeeded
snippets/csharp/System.Globalization/CultureInfo/CurrentCulture/changeculture1.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System.Globalization/CultureInfo/CurrentCulture/changeculture11.cs	✅Succeeded	View
snippets/csharp/System.Globalization/CultureInfo/CurrentCulture/currentculture.csproj	✅Succeeded
snippets/csharp/System.Globalization/JapaneseCalendar.ToDateTime/todatetime.csproj	✅Succeeded
snippets/csharp/System.IO/BinaryReader/Overview/BinaryReaderWriter.csproj	✅Succeeded
snippets/csharp/System.IO/Path/Combine/misc.csproj	✅Succeeded
snippets/csharp/System.IO/Path/Combine/pathcombine.cs	✅Succeeded	View
snippets/csharp/System.IO/Path/DirectorySeparatorChar/directoryseparatorchar.csproj	✅Succeeded
snippets/csharp/System.IO/Path/GetFullPath/getfullpath.csproj	✅Succeeded
snippets/csharp/System.IO/Path/IsPathRooted/ispathrooted.csproj	✅Succeeded
snippets/csharp/System.IO/Path/TryJoin/tryjoin.csproj	✅Succeeded
snippets/csharp/System.Net.Http/HttpClientHandler/Overview/system.net.http.httpclienthandler.csproj	✅Succeeded
snippets/csharp/System.Net.Http/HttpClientHandler/ServerCertificateCustomValidationCallback/program.cs	✅Succeeded	View
snippets/csharp/System.Net.Http/HttpClientHandler/ServerCertificateCustomValidationCallback/system.net.http.httpclienthandler-secure.csproj	✅Succeeded
snippets/csharp/System.Net.Http/SocketsHttpHandler/ConnectCallback/system.net.http.httpclienthandler-secure.csproj	✅Succeeded

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[gewarren]

Thanks @BillWagner!