Dedicated GA and Phase 9 Date alignment

Why is this change being made?

Currently, Phase 9 extends beyond a month (unlike the other phases) stretching into August. Additionally, the GA date is currently derived from the Phase 9 completion date and could be more clearly stated. This MR addresses these points and also shifts the GA date to 2023-08-02 mid-week, since the previous date 2023-08-05 falls on a Saturday.

Closes the discussion in https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/484#note_1252360500

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