Make storage limit effective dates more explicit

Why is this change being made?

We (Support) have received several tickets from confused customers trying to understand if/when they'll be impacted by upcoming storage limit changes for paid users. I am proposing this change based on what I think the FAQ is saying. If this is inaccurate, please make the appropriate clarifications to help our customers better understand this change.

I have adjusted the language to remove the mention that "changes will not take effect immediately." Since this is not an announcement page, and is instead an informational page, we should simply tell readers when and how the changes apply, so that the page will read the same regardless of the current date. This will avoid future confusion.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---