Update GitHub webhook and API schema for consistency

[cgalibern]

Description

This pull request includes the following updates to the GitHub webhook and API schema:

• Update GitHub webhook version to 0.2.1: Incremented the OpenAPI specification version, incorporating minor improvements or bug fixes while maintaining backward compatibility.
• Make GitHub API timestamps nullable: Added nullable properties to closed_at, updated_at, and merged_at fields in the schema for scenarios where these values might not exist.
• Update 'merged' field type: Changed the type of the merged field from bool to boolean to align with OpenAPI specifications and ensure consistency in type conventions.

This fix following 400 error when closed_at is null

{
  "detail": "None is not of type 'string' - 'pull_request.closed_at'",
 "status": 400,
 "title": "Bad Request",
 "type": "about:blank"
}