Add missing return value for Command's execute() methods (Symfony >= 4.4)

[sebb]

Fixes:

Return value of "Ambta\DoctrineEncryptBundle\Command\DoctrineEncryptStatusCommand::execute()" must be of the type int, "null" returned.
when using Symfony 5

Similar to #41 with the reviewers' suggestions implemented.

[tikoflano]

It would probably be better not to use self::FAILURE and self::SUCCESS but rather 1 and 0 in order to still support versions of Symonfy < 5.1

[r3hp1c]

Similar to another pull request that that were copied: DoctrineEncryptBundle#4

Please see these comments:

• Realllow usage of php 7.2+, CI using github actions, demoprojects and other improvements #65 (comment)
• Realllow usage of php 7.2+, CI using github actions, demoprojects and other improvements #65 (comment)

The new package that I hope will be maintained by more than 1 person