misc: Updated git description length policy to 72 chars

misc: Updated git description length policy to 72 chars

Advocating <=75 characters is non-standard. 72 characters is
recommended. Furthermore, within Gerrit, descriptions >72 characters
causes ugly line-wrapping. This commit updates this policy to
72 characters.

The corresponding change within the gem5 webpage:
https://gem5-review.googlesource.com/c/public/gem5-website/+/31134

Change-Id: Iaf542a241772f8e400d8f90624712de1a435d096
Issue-on: https://gem5.atlassian.net/browse/GEM5-665
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31135
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>