Fix detection of .NET Framework 4.8.1

[mattjohnsonpint]

Builds are failing because Github Actions runners now have .NET Framework 4.8.1 installed, and we don't have that in our version map.

Example failure:

https://github.com/getsentry/sentry-dotnet/runs/8105415270?check_suite_focus=true#step:6:556

Error: Expected string not to be <null>.

  Failed Sentry.Tests.PlatformAbstractions.RuntimeInfoTests.SetReleaseAndVersionNetFx_OnNetFx_NonNullReleaseAndVersion [13 ms]
  Error Message:
   Expected string not to be <null>.
  Stack Trace:
     at FluentAssertions.Execution.XUnit2TestFramework.Throw(String message)
   at FluentAssertions.Execution.AssertionScope.FailWith(Func`1 failReasonFunc)
   at FluentAssertions.Primitives.ReferenceTypeAssertions`2.NotBeNull(String because, Object[] becauseArgs)
   at Sentry.Tests.PlatformAbstractions.RuntimeInfoTests.SetReleaseAndVersionNetFx_OnNetFx_NonNullReleaseAndVersion() in D:\a\sentry-dotnet\sentry-dotnet\test\Sentry.Tests\PlatformAbstractions\RuntimeInfoTests.cs:line 51

[mattjohnsonpint]

FYI, according to the official docs, the only build number we should expect is 533325. However, in this comment a user mentions 533320 is possible also. Not sure how to validate that, but there doesn't seem to be any harm in adding both.