Update dependency NSubstitute to 6.1.0

[dependencyupdates]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
NSubstitute (source)	6.0.0 → 6.1.0

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Release Notes

nsubstitute/NSubstitute (NSubstitute)

v6.1.0

• [UPDATE] Revert nullability support from 6.0. This change caused a number of problems for callers (#​973, #​976), and
  trying to resolve these (#​976) revealed the need for more fundamental changes to NSubstitute to effectively support this.
  As a result, nullability is again disabled for public API.
  Huge thanks to @​jdb0123, @​Romfos, and @​Moha-sami for their PRs addressing this, and thanks for all those who raised
  issues. We also humbly award @​zvirja with one unlimited "I told you so" pass to use as he sees fit.

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