Fix C# warnings. (#21913)
### Description
<!-- Describe your changes. -->
Update some testing dependencies.
Fix various warnings. Mainly around documentation (existing) and unit
test usage (mainly resulting from xunit update).
Invalid angle brackets for generics in documentation were changed to use
curly braces based on
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/
> To refer to generic identifiers in code reference (cref) elements, you
can use either the escape characters (for example, cref="List<T>")
or braces (cref="List{T}"). As a special case, the compiler parses the
braces as angle brackets to make the documentation comment less
cumbersome to the author when referring to generic identifiers.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
