chore: Tighten contraints

[nielsenko]

Just tightens constraint to the actual lower bounds, ie. running

dart downgrade --tighten

using dart 3.8

This does not bump sdk to 3.9 (as I suggested on slack)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.45%. Comparing base (050026c) to head (8ae86a9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #333   +/-   ##
=======================================
  Coverage   91.45%   91.45%           
=======================================
  Files          97       97           
  Lines        3816     3816           
  Branches     1943     1943           
=======================================
  Hits         3490     3490           
  Misses        326      326           

Flag	Coverage Δ
relic_core	91.20% <ø> (ø)
relic_io	93.33% <ø> (ø)

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