Update README to flutter 3.3

[fzyzcjy]

:scroll: Description

:bulb: Motivation and Context

:green_heart: How did you test it?

:pencil: Checklist

• [x] I reviewed submitted code
• [ ] I added tests to verify changes
• [x] I updated the docs if needed
• [ ] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps

[fzyzcjy]

Flutter 3.3 says we no longer should run within a zone, so I did this code change.

[codecov-commenter]

Codecov Report

Base: 89.47% // Head: 89.47% // No change to project coverage :thumbsup:

Coverage data is based on head (b67d51e) compared to base (5a6e8fc). Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1002   +/-   ##
=======================================
  Coverage   89.47%   89.47%           
=======================================
  Files         106      106           
  Lines        3326     3326           
=======================================
  Hits         2976     2976           
  Misses        350      350           

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[marandaneto]

Makes sense but I'd only merge this once we work on https://github.com/getsentry/sentry-dart/issues/988

[marandaneto]

https://github.com/getsentry/sentry-dart/tree/main/flutter#usage