lttng: add-trigger: clarify terminology for log levels

lttng: add-trigger: clarify terminology for log levels

To eliminate ambiguity in the code, the terminology for log levels has
been updated. The previous terms "min" and "max" log levels have been
replaced with "least_severe" and "most_severe" respectively.

This change addresses the varying conventions across different logging
domains, where numerical values for severity can either increase or
decrease with severity. The new terminology provides clarity, making it
easier to understand the severity levels regardless of the logging
domain's convention.

Change-Id: Ie90bcc8e4c07b8b7437d9580e166141fae5c6d2f
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>