Upgrade the Flutter application to build and run on '>=3.1.1 <4.0.0' SDK version

[SzilvasiPeter]

Solved issues

• Upgraded SDK version: sdk: '>=3.1.1 <4.0.0'
• Null safety is ensured everywhere by 1) adding default values, 2) null-aware operator, and 3) required keyword for the parameters
• The smooth_star_rating package is replaced with the flutter_rating_bar package, which is null-safe
• Deprecated Widgets were replaced. For example: FlatButton with TextButton, RaisedButton with ElevatedButton, OutlineButton with OutlinedButton
• Deprecated parameters were replaced, for example, BottomNavigationBarItem.title to BottomNavigationBarItem.label
• Unused variables were removed
• PageTransition need aligment argument was fixed, alignment set to center
• Tested on Chromium and Ubuntu 20.04, application is compiled and working

Potential To-dos

I have added potential improvements for the application so other developers can make a PR.

1. No exhaustive testing #6 (comment)
2. No rating and favorite state are updated #7 (comment)
3. Not all tabs are implemented #8 (comment)
4. No authentication and user management #9 (comment)
5. No CI/CD pipeline for deployment #10 (comment)

Feel free to add more information to the issues by editing the comment or labeling (feature, test, deployment, etc.) them.