Add ksundberg as social backup for RP
Why is this change being made?
Follows up: !40472 (merged) based on discussions there.
Ensures people at least reading the source version of this information know of the correct backup.
The backup situation won't be obvious here: https://about.gitlab.com/handbook/marketing/blog/release-posts/managers/, if we want to mention a team Slack option, we might want to edit: https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/8a856dda49a9eeeb5a5a08ce124f941fd8dd3470/source/handbook/marketing/blog/release-posts/managers/index.html.haml#L9.
In the mean time, if this look ok to you, please forward on to @marcel.amirault.
Thanks!