x11-servers/xorg-server: Update to 1.20.13

Summary:
Changelogs to 1.20.12 in [1] and 1.20.13 in [2].

[1] https://lists.x.org/archives/xorg-announce/2021-July/003098.html
[2] https://lists.x.org/archives/xorg-announce/2021-July/003100.html

Reviewd by: x11 (manu,zeising)
Differential Revision: https://reviews.freebsd.org/D32539