housekeeping: Update Copyright Year

[worldbeater]

What kind of change does this PR introduce?

This PR updates ReactiveUI copyright year in the license header. Used a basic shell script and a manual json file update:
find -type f -exec ex -sc '%s/Copyright (c) 2020 .NET Foundation and Contributors/Copyright (c) 2021 .NET Foundation and Contributors/g' -cx {} ';'

[codecov]

Codecov Report

Merging #2677 (09bf1f5) into main (15f0e03) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2677   +/-   ##
=======================================
  Coverage   63.57%   63.57%           
=======================================
  Files         132      132           
  Lines        4733     4733           
=======================================
  Hits         3009     3009           
  Misses       1724     1724           

Impacted Files	Coverage Δ
.../ReactiveUI.Blend/FollowObservableStateBehavior.cs	0.00% <ø> (ø)
...activeUI.Blend/Platforms/net4/ObservableTrigger.cs	0.00% <ø> (ø)
src/ReactiveUI.Drawing/Registrations.cs	75.00% <ø> (ø)
...eUI.Fody.Helpers/ObservableAsPropertyExtensions.cs	41.93% <ø> (ø)
...tiveUI.Fody.Helpers/ReactiveDependencyAttribute.cs	0.00% <ø> (ø)
src/ReactiveUI.Testing/IBuilderExtensions.cs	77.27% <ø> (ø)
src/ReactiveUI.Testing/MessageBusExtensions.cs	0.00% <ø> (ø)
src/ReactiveUI.Testing/SchedulerExtensions.cs	54.71% <ø> (ø)
...rc/ReactiveUI.Winforms/ActivationForViewFetcher.cs	74.41% <ø> (ø)
...c/ReactiveUI.Winforms/ContentControlBindingHook.cs	0.00% <ø> (ø)
... and 18 more

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[github-actions]

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