Bump minimum supported automake version to 1.13

Bump minimum supported automake version to 1.13

since 1.13.4 is the earliest version of automake that we currently build
with regularly in the CI. Build tests are not currently run with 1.12 so
it is risky to claim supporting it.

Change-Id: I353a60f39c9ac8e1d279745b3225d64e40eab4cc
Signed-off-by: Erica Bugden <ebugden@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10586
CI-Build: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>