Remove references to 7.x and modernize tests to focus on 9.x

- [ ] [x-pack/lib/config_management/elasticsearch_source.rb](./blob/v9.2.0/x-pack/lib/config_management/elasticsearch_source.rb) drop 7.x code such as LegacyHiddenIndicesFetcher (and its tests)
- [ ] [x-pack/spec/config_management/elasticsearch_source_spec.rb](./blob/v9.2.0/x-pack/spec/config_management/elasticsearch_source_spec.rb) and [x-pack/spec/support/helpers.rb](./blob/v9.2.0/x-pack/spec/support/helpers.rb): move tests from 7.x+8.x to 9.x+8.x
- [ ] remove "since logstash 7.*" comments from files like [logstash-core/lib/logstash/config/mixin.rb](./blob/v9.2.0/logstash-core/lib/logstash/config/mixin.rb)
[logstash-core/lib/logstash/plugins/event_factory_support.rb](./blob/v9.2.0/logstash-core/lib/logstash/plugins/event_factory_support.rb)
[logstash-core/lib/logstash/plugin.rb](./blob/v9.2.0/logstash-core/lib/logstash/plugin.rb)
[logstash-core/lib/logstash/plugin_metadata.rb](./blob/v9.2.0/logstash-core/lib/logstash/plugin_metadata.rb)
[logstash-core/src/main/java/org/logstash/plugins/BasicEventFactory.java](./blob/v9.2.0/logstash-core/src/main/java/org/logstash/plugins/BasicEventFactory.java)
- [ ] [tools/benchmark-cli/src/main/java/org/logstash/benchmark/cli/LogstashInstallation.java](./blob/v9.2.0/tools/benchmark-cli/src/main/java/org/logstash/benchmark/cli/LogstashInstallation.java)
- [ ] [rakelib/artifacts.rake](./blob/v9.2.0/rakelib/artifacts.rake): e.g. `out.replaces << "logstash < 7.10.0"`
- [ ] [qa/README.md](./blob/v9.2.0/qa/README.md): e.g. `it_behaves_like "updated", logstash, from_release_branch="7.17"`
- [ ] [docs/reference/getting-started-with-logstash.md](./blob/v9.2.0/docs/reference/getting-started-with-logstash.md): e.g. `If you are upgrading from {{ls}} 7.11.x (or earlier),`
- [ ] [docs/reference/monitoring-internal-collection-legacy.md](./blob/v9.2.0/docs/reference/monitoring-internal-collection-legacy.md): e.g. `Deprecated in 7.9.0`
- [ ] [docs/reference/configuring-centralized-pipelines.md](./blob/v9.2.0/docs/reference/configuring-centralized-pipelines.md): e.g. `Wildcard in pipeline IDs is available starting with Elasticsearch 7.10`
- [ ] [docs/extend/](./blob/v9.2.0/docs/extend/): e.g. `7.6 introduced a *deprecation logger*`

Start with x-pack/lib/config_management/elasticsearch_source.rb and its related specs and helpers, then review the listed logstash-core Java/Ruby files, rakelib/artifacts.rake, QA README, and documentation pages for 7.x references. Verify that tests target 8.x and 9.x, legacy 7.x code and comments are removed, and all checklist locations have been updated.

Written by the indexing model from the issue text.