Relabel "Sync" to "Update fork" for pulling changes from an upstream repository

Proposal

Update the "Sync" button say Update fork in the fork widget.

Current	Proposal

Background

"Sync" could have an understanding that it is a two-way action. In the context of fetching commits, "sync" could mean that it brings the states of two things (two branches in this case) to the sync (same) state. It pulls in “behind” commits to the current fork, AND pushes “ahead” commits to the parent branch.

However in our case, that second part (pushing to the parent) is not part of “sync” operation. The push part is handled by creating a merge request.

Discussion was made in this issue https://gitlab.com/gitlab-org/gitlab/-/issues/392678