Remove deprecated SSL settings for HTTP output

[robbavey]

• All plugin changes that remove SSL settings should be in a new major release Mark deprecated SSL settings as obsolete #147
• Mark old SSL settings as obsolete
• Add details of change to the 9.0 breaking changes documentation in core. [Meta] Remove deprecated SSL settings from core and plugins elastic/logstash#16352
• Add breaking change entry to plugin changelog
• Verify that any integration tests in core that rely on SSL settings still work. Fix any broken integration tests
• Verify plugin documentation, remove any references to deprecated settings