refactor: update for compatibility with MTKv9

[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.

[AayushSabharwal]

Requires SciML/ModelingToolkit.jl#2480

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 54.12%. Comparing base (2da69d3) to head (27c1a1d).

Files	Patch %	Lines
src/Blocks/sources.jl	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
- Coverage   62.04%   54.12%   -7.92%     
==========================================
  Files          48       48              
  Lines        1449     1648     +199     
==========================================
- Hits          899      892       -7     
- Misses        550      756     +206     

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[ChrisRackauckas]

The standard library should all get units, but this is at least a non-breaking update so we can merge this and do the breaking one separately.