fix: use pdesystem for AbstractPDESolution SII

[AayushSabharwal]

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (855b485) 26.38% compared to head (72a8576) 5.27%.

Files	Patch %	Lines
src/solutions/solution_interface.jl	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #565 +/- ## ========================================== - Coverage 26.38% 5.27% -21.12% ========================================== Files 53 53 Lines 4086 4059 -27 ========================================== - Hits 1078 214 -864 - Misses 3008 3845 +837 ```

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