Fix: command option keep-outdated

[valentin-claras]

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Why

The option keep-outdated doesn't work as it is a string compared to a boolean. Whatever its value, it's not going to be strictly equal to false (from the command line).

How

We want to keep old value by default. And by putting the option, we expect to remove them. Since a boolean value is expected, change the option to be a boolean option (without value), with a name matching the opposite of its default behavior.