AVRO-4087: Support .NET 9

[zcsizmadia]

What is the purpose of the change

AVRO-4087

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Manually verified the change by building the website and checking the new redirect

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[zcsizmadia]

.NET 9 is released on November 12, 2024. This can be merged once that happens