docs: update NEWS.md with v9 update notes

Author: AayushSabharwal

NEWS.md

@@ -1,9 +1,34 @@
-# ModelingToolkit v8 Release Notes
+# ModelingToolkit v9 Release Notes
 
 ### Upgrade guide
 
-  - `connect` should not be overloaded by users anymore. `[connect = Flow]`
-    informs ModelingToolkit that particular variable in a connector ought to sum
-    to zero, and by default, variables are equal in a connection. Please check out
-    [acausal components tutorial](https://docs.sciml.ai/ModelingToolkit/stable/tutorials/acausal_components/)
-    for examples.
+- The function `states` is renamed to `unknowns`. In a similar vein:
+  - `unknown_states` is now `solved_unknowns`.
+  - `get_states` is `get_unknowns`.
+  - `get_unknown_states` is now `get_solved_unknowns`.
+- The default backend for using units in models is now `DynamicQuantities.jl` instead of
+  `Unitful.jl`.
+- ModelingToolkit.jl now exports common definitions of `t` (time independent variable)
+  and `D` (the first derivative with respect to `t`). Any models made using ModelingToolkit.jl
+  should leverage these common definitions. There are three variants:
+  - `t` and `D` use DynamicQuantities.jl units.
+  - `t_unitful` and `D_unitful` use Unitful.jl units.
+  - `t_nounits` and `D_nounits` are unitless.
+- `ODAEProblem` is deprecated in favor of `ODEProblem`.
+- Specifying the independent variable for an `ODESystem` is now mandatory. The `ODESystem(eqs)`
+  constructor is removed.
+- Systems must be marked as `complete` before creating `*Function`/`*FunctionExpr`/`*Problem`/
+  `*ProblemExpr`. Typically this involved using `@mtkbuild` to create the system or calling
+  `structural_simplify` on an existing system.
+- All systems will perform parameter splitting by default. Problems created using ModelingToolkit.jl
+  systems will have a custom struct instead of a `Vector` of parameters. The internals of this
+  type are undocumented and subject to change without notice or a breaking release. Parameter values
+  can be queried, updated or manipulated using SciMLStructures.jl or SymbolicIndexingInterface.jl.
+  This also requires that the symbolic type of a parameter match its assigned value. For example,
+  `@parameters p` will always use a `Float64` value for `p`. To use `Int` instead, use
+  `@parameters p::Int`. Array-valued parameters must be array symbolics; `@parameters p = [1.0, 2.0]`
+  is now invalid and must be changed to `@parameters p[1:2] = [1.0, 2.0]`. The index of a parameter
+  in the system is also not guaranteed to be an `Int`, and will instead be a custom undocumented type.
+  - To restore the old behavior, use `ModelingToolkit.@set sys.index_cache = nothing` before creating
+    a problem, and after calling `structural_simplify`.
+- Discrete-time system using `Difference` are unsupported. Instead, use the new `Clock`-based syntax.