[MAINT]: Remove preview API from Accept header

### Describe the need

The `antiope` preview [has graduated](https://developer.github.com/changes/2020-10-01-graduate-antiope-preview/) and is no longer needed.

The Accept header should be updated to remove it:

https://github.com/octokit/octokit.graphql.net/blob/0029ddeab2c020bb4efdb49e783bbb3cab08fc38/src/Octokit.GraphQL.Core/Connection.cs#L16

### SDK Version

Octokit.GraphQL

### API Version

0.4.0-beta

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

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[MAINT]: Remove preview API from Accept header #334

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[MAINT]: Remove preview API from Accept header #334

Description

Describe the need

SDK Version

API Version

Relevant log output

Code of Conduct

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

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