Chore: Widen package_info_plus constraint to support 10.x (1.8.1)

[bbbshah]

Fixes #40

Problem

once 1.8.0 pins package_info_plus ^9.0.0, which makes version solving fail for apps on newer Flutter that depend on package_info_plus 10.x:

Because once 1.8.0 depends on package_info_plus ^9.0.0 and no versions of once match >1.8.0 <2.0.0, once ^1.8.0 requires package_info_plus ^9.0.0.
So, because <app> depends on both package_info_plus ^10.2.1 and once ^1.8.0, version solving failed.

Changes

• Widen package_info_plus to >=9.0.0 <11.0.0 instead of a hard bump to ^10.0.0. once only uses PackageInfo.fromPlatform(), .version, and .buildNumber, which are identical across 9.x and 10.x — the 10.0.0 "breaking changes" were only raised platform minimums. The widened range means apps on older Flutter keep resolving 9.x while newer projects get 10.2.1, so nobody is forced to upgrade.
• Update flutter_lints to ^6.0.0 (dev-only) and raise the SDK floor to Dart 3.8 accordingly.
• Modernize the example app's SDK constraint (it was still >=2.16.1 <3.0.0, pre-Dart-3).
• Fix the analyzer findings that surfaced: assert on the previously-unused callbackCalled/fallbackCalled flags in runner_test.dart (slightly strengthening those tests), and use super parameters in the example.
• Bump version to 1.8.1 with a CHANGELOG entry (matching the 1.7.1 precedent for dependency-only releases).

Verification

• flutter analyze: no issues
• flutter test: all 57 tests pass
• dart pub publish --dry-run: clean
• End-to-end: a consumer with package_info_plus: ^10.2.1 + this branch resolves successfully (picks up 10.2.1); the error above is gone
• flutter pub outdated: all direct and dev dependencies at latest resolvable versions

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