[source-greenhouse] Use new v3 Greenhouse API as v1/v2 is deprecated and will be removed on August 31, 2026

### Connector Name

source-greenhouse

### Connector Version

0.7.28

### What step the error happened?

Configuring a new connector

### Relevant information

The source-greenhouse connector uses the Greenhouse API v1. The [offical API documentation](https://developers.greenhouse.io/harvest.html#authentication) alerts that both v1 and v2 APIs are deprecated and will be removed on August 31, 2026.

The source connector must be updated to use the [new v3 API](https://harvestdocs.greenhouse.io/reference/generate-token) instead, which introduces OAuth authentication.

### Relevant log output

```shell

```

### Contribute

- [ ] Yes, I want to contribute

---
**Internal Tracking:** https://github.com/airbytehq/oncall/issues/13145

Start with the source-greenhouse connector and read the linked Greenhouse v3 API documentation, especially the token-generation and OAuth requirements. The work is done when the connector uses the v3 API and OAuth authentication instead of the deprecated v1/v2 APIs, with configuring a new connector still working.

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