This commit updates SSL settings to be marked as obsolete:
- Replace `ssl` with `ssl_enabled`
- Replace `ca_file` with `ssl_certificate_authorities`
- Replace `ssl_certificate_verification` with `ssl_verification_mode`

`setup_ssl_params!` has been updated to only handle SSL inference
when not explicitly configured.

All changes have been updated in tests and in docs. The preparation for
releasing a new major version is also included.

This commit updates the documentation to follow the pattern established in
logstash-plugins/logstash-output-http#147 for
documenting obsolete options.