Docs/secrets sync ssrf private endpoints

[pixambi]

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The Secrets Sync update in 1.17.3 introduced SSRF protection that blocks private IP ranges, affecting users accessing secret stores through private endpoints. Adding documentation about the change

Description

This PR adds documentation to the 1.17.x upgrade guide about a breaking change in Secrets Sync introduced in version 1.17.3. The documentation:

• Explains the new SSRF protection mechanism added to Secrets Sync
• Provides example error messages users might encounter
• Lists current options for users affected by this change

This change helps users understand why their Secrets Sync operations might fail after upgrading to 1.17.3 and what options are available to them.

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• 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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CI Results:
All Go tests succeeded! ✅

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[yhyakuna]

Added minor suggestions.

[pixambi]

N/A

[pixambi]

accepted all changes