housekeeping: Update to C# 9 syntax

[glennawatson]

No description provided.

[codecov]

Codecov Report

Merging #182 (5abaa90) into main (f2933a0) will increase coverage by 0.37%.
The diff coverage is 71.79%.

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
+ Coverage   62.55%   62.93%   +0.37%     
==========================================
  Files          15       15              
  Lines         900      893       -7     
==========================================
- Hits          563      562       -1     
+ Misses        337      331       -6     

Impacted Files	Coverage Δ
...lidation/Extensions/ValidationContextExtensions.cs	87.50% <ø> (ø)
.../TemplateGenerators/PropertyValidationGenerator.cs	0.00% <0.00%> (ø)
....Validation/Comparators/ValidationStateComparer.cs	37.50% <66.66%> (ø)
...eactiveUI.Validation/Collections/ValidationText.cs	78.94% <72.72%> (-2.01%)	⬇️
...veUI.Validation/Components/ObservableValidation.cs	92.50% <77.77%> (ø)
.../ReactiveUI.Validation/Helpers/ValidationHelper.cs	95.45% <85.71%> (-4.55%)	⬇️
...UI.Validation/Components/BasePropertyValidation.cs	93.42% <100.00%> (-0.09%)	⬇️
...eactiveUI.Validation/Contexts/ValidationContext.cs	100.00% <100.00%> (+7.31%)	⬆️
...ation/Extensions/ValidatableViewModelExtensions.cs	71.96% <100.00%> (ø)
...dation/Extensions/ValidatesPropertiesExtensions.cs	83.33% <100.00%> (ø)
... and 5 more

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