SECURITY: Drop "patch level" from the supported version number list

This commit drops the "patch level" from the supported version numbers
listed in the security policy document because:

1. It currently does not convey any meaningful information (simply set
   to "0") and is really unnecessary because the support status is
   defined at the "minor" version level.

2. Updating the security policy document whenever there is a new "patch
   release" is arguably too noisy and does not add much value.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>