avoid X509Certificate.getIssuerDN (deprecated since java 16)

``` [warn] /home/runner/work/pekko-connectors/pekko-connectors/google-common/src/main/scala/org/apache/pekko/stream/connectors/google/http/ForwardProxyHttpsContext.scala:32:29: method getIssuerDN in class X509Certificate is deprecated (since 16) [warn] val alias = certificate.getIssuerDN.getName ```