rearchive_reports_in_past_last_n_months already has last_n in the name but still expects to configure last4 as value for example. Goal of this issue is that

• If only a number is configured, we skip the substr https://github.com/matomo-org/matomo/blob/4.2.1/core/Archive/ArchiveInvalidator.php#L799
• We also no longer hard code the 4 (which comes from strlen('last')) but would instead do something like str_replace('last', '', $lastNMonthsToInvalidate) which be more readable.

Note same code seems to be also in 2 places in Custom Reports where it would needs to be fixed too