Update version-specific.md
Add the appropriate "if not defined" block on the examples for 'CIVICRM_CRED_KEYS' and 'CIVICRM_SIGN_KEYS' to match https://docs.civicrm.org/sysadmin/en/latest/setup/secret-keys/#generate-secrets
Add the appropriate "if not defined" block on the examples for 'CIVICRM_CRED_KEYS' and 'CIVICRM_SIGN_KEYS' to match https://docs.civicrm.org/sysadmin/en/latest/setup/secret-keys/#generate-secrets