Update FlexUnits dependency version in Project.toml

[Deduction42]

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.

[Deduction42]

This simply updates the FlexUnits.jl compat entry. Breaking changes only deal with direct Dimensions manipulations which do not occur in the extension.