Attributes broken net 9

[atlemann]

Attribute usage doesn't compile on net9

type ImportArgs =
    | [<Mandatory>] Name of name: string

Gives me this error for the Mandatory attribute:

error FS0842: This attribute is not valid for use on this language element

[atlemann]

Funnily enough, if I copy the attribute code and put it above the ImportArgs, it stops complaining

/// Demands at least one parsed result for this argument; a parse exception is raised otherwise.
[<AttributeUsage(AttributeTargets.Class ||| AttributeTargets.Method ||| AttributeTargets.Property, AllowMultiple = false)>]
type MandatoryAttribute () = inherit Attribute ()

type ImportArgs =
    | [<Mandatory>] Name of name: string

[eiriktsarpalis]

Tracked by dotnet/fsharp#18298