housekeeping: Update copyright year

[worldbeater]

What kind of change does this PR introduce?

This updates copyright year in source files using a magic spell from reactiveui/ReactiveUI#2677 (comment)

[codecov]

Codecov Report

Merging #345 (a2ea5fd) into main (6517cd8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #345   +/-   ##
=======================================
  Coverage   66.87%   66.87%           
=======================================
  Files          31       31           
  Lines         809      809           
=======================================
  Hits          541      541           
  Misses        268      268           

Impacted Files	Coverage Δ
src/Sextant.Mocks/AbstractViewModel.cs	50.00% <ø> (ø)
src/Sextant.Mocks/NavigableViewModelMock.cs	90.00% <ø> (ø)
src/Sextant.Mocks/NavigatedMock.cs	0.00% <ø> (ø)
src/Sextant.Mocks/NavigationViewMock.cs	55.00% <ø> (ø)
src/Sextant.Mocks/NullViewModelMock.cs	0.00% <ø> (ø)
src/Sextant.Mocks/PageView.cs	0.00% <ø> (ø)
src/Sextant.Mocks/ParameterViewModel.cs	100.00% <ø> (ø)
src/Sextant.Plugins.Popup/PopupNavigationEvent.cs	70.00% <ø> (ø)
src/Sextant.Plugins.Popup/PopupViewStackService.cs	100.00% <ø> (ø)
...Sextant.Plugins.Popup/PopupViewStackServiceBase.cs	88.43% <ø> (ø)
... and 21 more

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