Update eck_version & eck_release_branch in eck-attributes for upcoming (#8303)

2.16 release

Signed-off-by: Michael Montgomery <[email protected]>

Author: naemono

docs/eck-attributes.asciidoc

@@ -1,6 +1,6 @@
-:eck_version: 2.15.0
+:eck_version: 2.16.0
 :eck_crd_version: v1
-:eck_release_branch: 2.15
+:eck_release_branch: 2.16
 :eck_github: https://github.com/elastic/cloud-on-k8s
 :eck_resources_list: Elasticsearch, Kibana, APM Server, Enterprise Search, Beats, Elastic Agent, Elastic Maps Server, and Logstash