Fix links to upcoming releases page, standardize some language

This MR fixes a few links which were pointing to the old future releases page (now upcoming releases), as well as standardizing some language.

I left a alone a few items, as there is a difference in meaning between upcoming and future, namely that upcoming implies it will be happening soon:

• Upcoming: forthcoming, about to happen
• Future: existing or occurring at a late time

Places I left "future releases" alone:

• On the product handbook we differentiate between upcoming releases and future releases, one being sooner than the other: https://about.gitlab.com/handbook/product/#managing-upcoming-releases. I think this is a decent way of talking about the near term releases, which we need to work together with other stakeholders to plan, and further out releases which haven't gone through this diligence yet.
• Blog posts, in many instances talking about an iteration in a future release.