Improve composer files

[gmponos]

• Removed composer.lock file. In general you will notice that packages do not submit composer.lock file. Check symfonys package for example.
• Running composer validate shows some errors

The lock file is not up to date with the latest changes in composer.json, it is recommended that you run `composer update`.

• Added keywords.
• Added license
• Require a minimum PHP version inside the package since it can be easily overridden.

Please also consider the following:

Rename the composer name to: thanos-kontos/greek-validators-symfony inside the composer.json it is as thanosKontos/greek-validators-symfony

From composer validate:

Name "thanosKontos/greek-validators-symfony" does not match the best practice (e.g. lower-cased/with-dashes). We suggest using "thanos-kontos/greek-validators-symfony" instead. As such you will not be able to submit it to Packagist.