Bug 28156: Rename Koha::Account::Line->renewal with is_renewal

Bug 28156: Rename Koha::Account::Line->renewal with is_renewal

It's a boolean, it must be named is_*

Test plan:
Confirm that
  prove t/db_dependent/Koha/Account/Line.t
is still returning green

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 491147896d5cced31861243b862e403f09d62d98)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>