Updates release date in the instructions to create the monthly doc version

What does this MR do and why?

This MR updates the scheduled release date in https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/releases.md.

Previously, GitLab releases were scheduled for the 22nd of the month. As of October, GitLab releases are now scheduled on the third Thursdays every month. No more weekend/holiday releases.

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