package/webkitgtk: security bump to version 2.28.3

package/webkitgtk: security bump to version 2.28.3

This is a minor release which provides fixes for CVE-2020-9800,
CVE-2020-9802, CVE-2020-9803, CVE-2020-9805, CVE-2020-9806,
CVE-2020-9807, CVE-2020-9843, CVE-2020-9850, and CVE-2020-13753.

Updating from 2.28.2 also brings in the usual batch of fixes, including
important improvements to threading in the media player. Full release
notes can be found at:

  https://webkitgtk.org/2020/07/09/webkitgtk2.28.3-released.html

A detailed security advisory can be found at:

  https://webkitgtk.org/security/WSA-2020-0006.html

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>