Fix build

[zakaluka]

Proposed Changes

Pin .NET Core version to 3.1.201 due to incompatibility with FAKE packages.

Types of changes

What types of changes does your code introduce to zn-llrbtree?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

N/A