ES6 template syntax

[CaringDev]

Does this issue relate to a new feature or an existing bug?

• Bug
• New Feature

What version of Serilog.Sinks.Elasticsearch is affected? Please list the related NuGet package.
6.5.0

What is the target framework and operating system? See target frameworks & net standard matrix.

• netCore 2.0
• netCore 1.0
• 4.7
• 4.6.x
• 4.5.x

Please describe the current behavior?
Index Template for v6 uses old "template" field for index matching.

Please describe the expected behavior?
ES Sink uses the new "index_patterns" field when generating a template.

If the current behavior is a bug, please provide the steps to reproduce the issue and if possible a minimal demo of the problem
Using settings as follows:

    // ...
    AutoRegisterTemplate = true,
    AutoRegisterTemplateVersion = AutoRegisterTemplateVersion.ESv6,
    // ...

the sink tries to post a template according to template_v6.json leading to an error:

{
  "error": {
    "root_cause": [
      {
        "type": "action_request_validation_exception",
        "reason": "Validation Failed: 1: index patterns are missing;"
      }
    ],
    "type": "action_request_validation_exception",
    "reason": "Validation Failed: 1: index patterns are missing;"
  },
  "status": 400
}