Obfuscate secret token after removing Fleet's ability to read secrets

Fleet/Kibana now supports secrets in the Kibana UI, as described in https://github.com/elastic/kibana/issues/154715#issuecomment-1678430435. 
We should set `secret: true` for `secret-token` in the apm package (https://github.com/elastic/apm-server/blob/main/apmpackage/apm/manifest.yml#L52).  