Bump shared_preferences from 2.0.18 to 2.0.20

Bumps [shared_preferences](https://github.com/flutter/packages/tree/main/packages/shared_preferences) from 2.0.18 to 2.0.20.
- [Release notes](https://github.com/flutter/packages/releases)
- [Commits](https://github.com/flutter/packages/commits/shared_preferences-v2.0.20/packages/shared_preferences)

---
updated-dependencies:
- dependency-name: shared_preferences
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]