Backport of website: restrict node engines to <= 16.x into release/1.12.x

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

Backport

This PR is auto-generated from #18077 to be assessed for backporting due to the inclusion of the label backport/1.12.x.

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

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Restrict the node engines declaration for the website so it only uses up to 16.x.

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Overview of commits

• 01b6b1fd19ba26b758037d551714c6c987063fc6

[brkalow]

Vercel deploy failure is fixed by: https://github.com/hashicorp/vault/pull/18078