Remove deprecated SSL settings from Elasticsearch input plugin

[robbavey]

• [ ] All plugin changes that remove SSL settings should be in a new major release of the plugin
• [x] Mark old SSL settings as obsolete
• [x] Add breaking change entry to plugin changelog
• [ ] Add details of change to the 9.0 breaking changes documentation in core. - [] Verify that any integration tests in core that rely on SSL settings still work. Fix any broken integration tests
• [x] Verify plugin documentation, remove any references to deprecated settings

[donoghuc]

NOTE: No integration tests in core explicitly configure this plugin.