package/harfbuzz: bump to version 2.6.8

package/harfbuzz: bump to version 2.6.8

- update home page URL (old one is redirected to github)
- change download to new location (github)
- update license hash because of copyright year update:

diff COPING:
-Copyright © 2010,2011,2012,2013,2014,2015,2016,2017,2018,2019  Google, Inc.
-Copyright © 2019  Facebook, Inc.
+Copyright © 2010,2011,2012,2013,2014,2015,2016,2017,2018,2019,2020  Google, Inc.
+Copyright © 2018,2019,2020  Ebrahim Byagowi
+Copyright © 2019,2020  Facebook, Inc.

note: newer versions (2.7.x) require a migration to meson
Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>