Fix minimum Julia version requirement from 1.6 to 1.10

The current minimum version bound of Julia >= 1.6 is incorrect.

## Evidence

1. **DifferentialEquations.jl** requires Julia >= 1.10 (as of v8.0.0, Oct 2025)
2. **ModelingToolkit.jl v9+** requires Julia >= 1.9 (as of v9.0.0, Feb 2024)
3. The current diffeqr code uses `complete(odesys, split=false)` which requires
   ModelingToolkit.jl v9.0.0+ (introduced in PR #2436)

## Breaking Changes Timeline

- **Feb 2024**: ModelingToolkit v9.0.0 required systems to be `complete`d
  before creating problems, requiring Julia 1.9+
- **Oct 2025**: DifferentialEquations.jl v8.0.0 bumped to Julia 1.10+

## Impact

Users with Julia 1.6-1.9 will encounter errors when trying to use diffeqr
with current versions of DifferentialEquations.jl. This change correctly
documents the actual minimum version required.

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: claude

DESCRIPTION

@@ -14,7 +14,7 @@ Depends: R (>= 3.4.0)
 Encoding: UTF-8
 License: MIT + file LICENSE
 URL: https://github.com/SciML/diffeqr
-SystemRequirements: Julia (>= 1.6), DifferentialEquations.jl, ModelingToolkit.jl
+SystemRequirements: Julia (>= 1.10), DifferentialEquations.jl, ModelingToolkit.jl
 Imports:
   JuliaCall
 RoxygenNote: 7.1.1