bump 7.x branch to 7.9.0-SNAPSHOT

[jmlrt]

This commit bump the 7.x branch which will be dedicated to the 7.x.x releases for the Helm Charts. It will allow to:

• test this branch with the daily Stack Docker images 7.9.0-SNAPSHOT via dedicated Jenkins jobs
• test the staging 7.9.x Docker images before a release

This is also backporting some updates in README files that were made in other branches (see #660) and fixing the links to Elastic documentation

[fatmcgav]

LGTM

[jmlrt]

Logstash chart test with 7.9.0-SNAPSHOT image and default values is failing with the following error:

[2020-06-12T11:46:07,182][ERROR][logstash.monitoringextension.pipelineregisterhook] Failed to set up the metrics pipeline {:message=>"With the default xpack.monitoring.elasticsearch.username, xpack.monitoring.elasticsearch.password must be set", :backtrace=>["/usr/share/logstash/x-pack/lib/helpers/elasticsearch_options.rb:207:in `validate_authentication!'", "/usr/share/logstash/x-pack/lib/helpers/elasticsearch_options.rb:50:in `es_options_from_settings'", "/usr/share/logstash/x-pack/lib/helpers/elasticsearch_options.rb:41:in `es_options_from_settings_or_modules'", "/usr/share/logstash/x-pack/lib/monitoring/monitoring.rb:201:in `generate_pipeline_config'", "/usr/share/logstash/x-pack/lib/monitoring/monitoring.rb:180:in `setup_metrics_pipeline'", "/usr/share/logstash/x-pack/lib/monitoring/monitoring.rb:142:in `after_agent'", "org/logstash/execution/EventDispatcherExt.java:91:in `execute'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:388:in `execute'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:268:in `run'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/clamp-0.6.5/lib/clamp/command.rb:132:in `run'", "/usr/share/logstash/lib/bootstrap/environment.rb:88:in `<main>'"]}
[2020-06-12T11:46:07,185][FATAL][logstash.runner          ] An unexpected error occurred! {:error=>#<ArgumentError: With the default xpack.monitoring.elasticsearch.username, xpack.monitoring.elasticsearch.password must be set>, :backtrace=>["/usr/share/logstash/x-pack/lib/helpers/elasticsearch_options.rb:207:in `validate_authentication!'", "/usr/share/logstash/x-pack/lib/helpers/elasticsearch_options.rb:50:in `es_options_from_settings'", "/usr/share/logstash/x-pack/lib/helpers/elasticsearch_options.rb:41:in `es_options_from_settings_or_modules'", "/usr/share/logstash/x-pack/lib/monitoring/monitoring.rb:201:in `generate_pipeline_config'", "/usr/share/logstash/x-pack/lib/monitoring/monitoring.rb:180:in `setup_metrics_pipeline'", "/usr/share/logstash/x-pack/lib/monitoring/monitoring.rb:142:in `after_agent'", "org/logstash/execution/EventDispatcherExt.java:91:in `execute'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:388:in `execute'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:268:in `run'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/clamp-0.6.5/lib/clamp/command.rb:132:in `run'", "/usr/share/logstash/lib/bootstrap/environment.rb:88:in `<main>'"]}
[2020-06-12T11:46:07,247][ERROR][org.logstash.Logstash    ] java.lang.IllegalStateException: Logstash stopped processing because of an error: (SystemExit) exit

This seems related to a change in Logstash 7.9.x.

All the other tests are successfull. I'll merge this PR to have 7.9.0-SNAPSHOT tests (even if they are failing because of Logstash) and create a new issue to address the Logstash error.