Backport of Auto-rolling billing start docs PR into release/1.17.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #27926 to be assessed for backporting due to the inclusion of the label backport/1.17.x.

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Warning automatic cherry-pick of commits failed. If the first commit failed,
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The person who merged in the original PR is:
@akshya96
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/vault/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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Description

What does this PR do?
Docs changes for auto-roll billing start date
JIRA: https://hashicorp.atlassian.net/browse/VAULT-26332
RFC: https://docs.google.com/document/d/1p7s1J_3glPXnBeY-mM-U_pmeLyagYtgT5zgX_5BRrbM/edit?usp=sharing
Approved docs PR: #27685

TODO only if you're a HashiCorp employee

• Labels: If this PR is the CE portion of an ENT change, and that ENT change is
  getting backported to N-2, use the new style backport/ent/x.x.x+ent labels
  instead of the old style backport/x.x.x labels.
• Labels: If this PR is a CE only change, it can only be backported to N, so use
  the normal backport/x.x.x label (there should be only 1).
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

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Overview of commits

• e1d3c32

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CI Results:
All Go tests succeeded! ✅

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