Renew Vault intermediate authority certificate

Summary:
Provide an helper script to regenerate the intermediate authority:

• create a CSR
• sign it with root certificate
• assign it to pki_vault
• update signing role

Bootstrap instructions updated with new multi-issuer procedure.

References:

• https://developer.hashicorp.com/vault/tutorials/pki/pki-engine
• https://developer.hashicorp.com/vault/api-docs/secret/pki

Ref T2112

Test Plan: Renewed 2025-09-10 ecrtificate with pki-intermediate-regenerate.sh

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2112

Differential Revision: https://devcentral.nasqueron.org/D3657