Annotate or get rid of the null-forgiving operators in PropertyGrid.cs and related files.

This is a follow up on https://github.com/dotnet/winforms/pull/10110#discussion_r1360819144.

Investigate if https://github.com/dotnet/winforms/blob/f7d6850caa53f62816d977e737d6cb2a537f0d9a/src/System.Windows.Forms/src/System/Windows/Forms/Controls/PropertyGrid/PropertyGrid.cs#L3770 can be refactored to convince nullability analyzers that fields that are allocated in this method are not null. This would allow us to get rid of the null-forgiving operators. If not realistic, then then each null-forgiving operator should be annotated.