Use IReadOnlySet on .NET 5

[bart-degreed]

Description

We are currently using IReadOnlyCollection<T> and ISet<T> in various places throughout the codebase.

JADNC currently requires .NET Core 3.1+.

In .NET 5, IReadOnlySet<T> was added. When we upgrade JADNC to .NET 5, we should consider to use the newer collection types like this one.

[bart-degreed]

Alternatively, we could use the existing IImmutableSet<T> for SparseFieldSetExpression.Attributes, which sounds more appropriate, given the immutable nature of QueryExpression trees. Note, however, that order is no longer preserved in that case. That breaks some existing tests, but is fixable.

Tracked at #1027.