Add upper SDK constraint to 3.20.0

[fujidaiti]

Temporarily set the upper constraint of the Flutter SDK version to 3.20.0. This is necessary because the current SDK used to analyze packages on pub.dev is 3.20.0-1.1.pre, which has a breaking change in the PageView class that is not compatible with the current version of this package. As a result, static analysis errors occur on pub.dev.

[fujidaiti]

Won't publish this version because:

Package validation found the following potential issue:

• The Flutter constraint should not have an upper bound.
  In your pubspec.yaml the constraint is currently >=3.10.6 < 3.20.0.

  You can replace that with just the lower bound: >=3.10.6.