housekeeping: Update Copyright Year

[worldbeater]

What kind of change does this PR introduce?

This PR updates ReactiveUI.Validation copyright year in the license header. Based on reactiveui/ReactiveUI#2677

[codecov]

Codecov Report

Merging #221 (aef51d2) into main (3c1fef1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #221   +/-   ##
=======================================
  Coverage   63.04%   63.04%           
=======================================
  Files          15       15           
  Lines         893      893           
=======================================
  Hits          563      563           
  Misses        330      330           

Impacted Files	Coverage Δ
...eactiveUI.Validation/Collections/ValidationText.cs	78.94% <ø> (ø)
....Validation/Comparators/ValidationStateComparer.cs	37.50% <ø> (ø)
...UI.Validation/Components/BasePropertyValidation.cs	94.73% <ø> (ø)
...veUI.Validation/Components/ObservableValidation.cs	92.50% <ø> (ø)
...eactiveUI.Validation/Contexts/ValidationContext.cs	100.00% <ø> (ø)
...veUI.Validation/Extensions/ExpressionExtensions.cs	100.00% <ø> (ø)
...ation/Extensions/ValidatableViewModelExtensions.cs	71.96% <ø> (ø)
...dation/Extensions/ValidatesPropertiesExtensions.cs	83.33% <ø> (ø)
...lidation/Extensions/ValidationContextExtensions.cs	87.50% <ø> (ø)
...iveUI.Validation/Formatters/SingleLineFormatter.cs	100.00% <ø> (ø)
... and 5 more

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