Remove experimental remarks for subcell limiting

[bennibolm]

The current support for subcell limiting is around for quite some time and runs stable. I think it's time to remove the "experimental" remarks.
I added this to NEWS and added a comment in the README (the latter is of course to be discussed and I can remove it again).

[github-actions]

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[bennibolm]

For now, I kept the experimental status for the get_boundary_outer_state routine in src/solvers/dgsem_tree/dg_2g_subcell_limiters.jl since in my opinion this feature is still possible to be changed in the future.

Trixi.jl/src/solvers/dgsem_tree/dg_2d_subcell_limiters.jl

Lines 474 to 500 in cde00a8

	"""
	get_boundary_outer_state(u_inner, t,
	boundary_condition::BoundaryConditionDirichlet,
	orientation_or_normal, direction,
	equations, dg, cache, indices...)
	For subcell limiting, the calculation of local bounds for non-periodic domains requires the boundary
	outer state. This function returns the boundary value for [`BoundaryConditionDirichlet`](@ref) at
	time `t` and for node with spatial indices `indices` at the boundary with `orientation_or_normal`
	and `direction`.
	Should be used together with [`TreeMesh`](@ref) or [`StructuredMesh`](@ref).
	!!! warning "Experimental implementation"
	This is an experimental feature and may change in future releases.
	"""
	@inline function get_boundary_outer_state(u_inner, t,
	boundary_condition::BoundaryConditionDirichlet,
	orientation_or_normal, direction,
	equations, dg, cache, indices...)
	(; node_coordinates) = cache.elements
	x = get_node_coords(node_coordinates, equations, dg, indices...)
	u_outer = boundary_condition.boundary_value_function(x, t, equations)
	return u_outer
	end
	end # @muladd

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (744e9a9) to head (eb84655).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2084   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files         470      470           
  Lines       37492    37492           
=======================================
  Hits        36114    36114           
  Misses       1378     1378           

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[DanielDoehring]

Xref #2078

[DanielDoehring]

Any plans to get this ready for ##1997 ?

[bennibolm]

Any plans to get this ready for ##1997 ?

From my side, it would be cool to include it. I mean, there is probably not much more to do anyway.

[bennibolm]

Thanks @ranocha.
Moreover, are you guys fine with keeping the experimental status for get_boundary_outer_state as mentioned above?

[ranocha]

Yes, thanks