chore: Added override to build locally with 2022

[bitsandfoxes]

From testing this locally it worked fine until I tried building with 2022.

error CS1705: Assembly 'UnityEngine' with identity 'UnityEngine, uses 'netstandard, Version=2.1.0.0", 
which has a higher version than referenced assembly 'netstandard' 
with identity 'netstandard, Version=2.0.0.0' [/Users/bitfox/Workspace/sentry-unity/src/Sentry.Unity/Sentry.Unity.csproj]

#skip-changelog

[mattjohnsonpint]

We'll want a .NET Standard 2.1 version of Sentry to be used as well. Adding this in getsentry/sentry-dotnet#2328. Not sure if there's any additional work to ensure it's used for Unity 2022 or not.

[mattjohnsonpint]

... probably not, since it's just a ProjectReference