Thankfully they are actively working towards upgrading, Unity 6.8 (they're currently on 6.4) is supposed to move fully towards CoreCLR, and removing Mono. We'll then finally be able to move to C# 14 (from C# 9, which came out in 2020), as well as use newer .NET functionality.
One annoying piece of Unity's CoreCLR plan is there is no plan to upgrade IL2CPP (Unity's AOT compiler) to use a better garbage collector. It will continue to use Boehm GC, which is so much worse for games.
They just haven't announced any plans to do so yet. They might one day.
They will not be using .NET AOT probably ever though. Unity's AOT basically supports full C# (reflection etc) while .NET opted to restrict it and lean more on generated code.
