Clean a bunch of C# warnings

- `[Obsolete]` tag generated in the middle of documentation comments
- Potential `null` values in generators
- Obsolete call to `GetEditorInterface()`
- We don't want `Godot.Collections.Array` to end with `Collection`
- A few culture specifications and use of `AsSpan` instead of `SubString` in `StringExtensions`
- Disable CA1716 in GodotSharp

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/CodeAnalysisAttributes.cs

@@ -0,0 +1,7 @@
+namespace System.Diagnostics.CodeAnalysis
+{
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue)]
+    public sealed class NotNullAttribute : Attribute
+    {
+    }
+}