CompatHelper: bump compat for ModelingToolkit to 9 for package docs, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the ModelingToolkit package from 8 to 8, 9 for package docs.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[schillic]

This PR should not be merged as long as v9 of ModelingToolkit cannot be tested (there were several breaking changes). See below (cause: ReachabilityAnalysis holds back TaylorIntegration).

(docs) pkg> add ModelingToolkit@9
   Resolving package versions...
ERROR: Unsatisfiable requirements detected for package SymbolicIndexingInterface [2efcf032]:
 SymbolicIndexingInterface [2efcf032] log:
 ├─possible versions are: 0.1.0-0.3.7 or uninstalled
 ├─restricted by compatibility requirements with ModelingToolkit [961ee093] to versions: 0.3.1-0.3.7
 │ └─ModelingToolkit [961ee093] log:
 │   ├─possible versions are: 0.0.1-9.0.1 or uninstalled
 │   └─restricted to versions 9 by an explicit requirement, leaving only versions: 9.0.0-9.0.1
 └─restricted by compatibility requirements with RecursiveArrayTools [731186ca] to versions: 0.1.0-0.2.2 — no versions left
   └─RecursiveArrayTools [731186ca] log:
     ├─possible versions are: 0.16.0-3.10.0 or uninstalled
     ├─restricted by compatibility requirements with ReachabilityAnalysis [1e97bd63] to versions: 2.0.0-3.10.0
     │ └─ReachabilityAnalysis [1e97bd63] log:
     │   ├─possible versions are: 0.1.0-0.24.0 or uninstalled
     │   ├─restricted to versions 0.18.6-0.24 by ReachabilityModels [20aa409e], leaving only versions: 0.18.6-0.24.0
     │   │ └─ReachabilityModels [20aa409e] log:
     │   │   ├─possible versions are: 0.1.0 or uninstalled
     │   │   └─ReachabilityModels [20aa409e] is fixed to version 0.1.0
     │   └─restricted to versions 0.22-0.24 by an explicit requirement, leaving only versions: 0.22.0-0.24.0
     ├─restricted by compatibility requirements with ModelingToolkit [961ee093] to versions: 2.3.0-3.10.0
     │ └─ModelingToolkit [961ee093] log: see above
     ├─restricted by compatibility requirements with TaylorIntegration [92b13dbe] to versions: 2.0.0-2.38.10, leaving only versions: 2.3.0-2.38.10
     │ └─TaylorIntegration [92b13dbe] log:
     │   ├─possible versions are: 0.4.0-0.14.7 or uninstalled
     │   └─restricted by compatibility requirements with ReachabilityAnalysis [1e97bd63] to versions: 0.9.0-0.10.0
     │     └─ReachabilityAnalysis [1e97bd63] log: see above
     └─restricted by compatibility requirements with SciMLBase [0bca4576] to versions: 2.33.0-3.10.0, leaving only versions: 2.33.0-2.38.10
       └─SciMLBase [0bca4576] log:
         ├─possible versions are: 1.0.0-2.26.2 or uninstalled
         ├─restricted by compatibility requirements with ModelingToolkit [961ee093] to versions: 2.0.1-2.26.2
         │ └─ModelingToolkit [961ee093] log: see above
         └─restricted by compatibility requirements with RecursiveArrayTools [731186ca] to versions: 1.0.0-2.10.0 or uninstalled, leaving only versions: 2.0.1-2.10.0
           └─RecursiveArrayTools [731186ca] log: see above