billing: update usage terminology (#23443)

## Description
- Remove "credits" and "budget" from docs, to use term "usage"

## Related issues or tickets

https://docker.atlassian.net/browse/ENGDOCS-2934?atlOrigin=eyJpIjoiMDIzNGJlNzFiZjlhNGI4NjhkOGVkNjExNzA0MzlmZTYiLCJwIjoiaiJ9

content/manuals/offload/quickstart.md

@@ -36,7 +36,7 @@ up](https://www.docker.com/products/docker-offload/) and subscribe.
    ```
 
 3. When prompted, select your account to use for Docker Offload. This account
-   will consume credits for your Docker Offload usage.
+   will consume Docker Offload usage.
 
 4. When prompted, select whether to enable GPU support. If you choose to enable
    GPU support, Docker Offload will run in an instance with an NVIDIA L4 GPU,

content/manuals/offload/usage.md

@@ -10,9 +10,9 @@ keywords: cloud, usage, cloud minutes, shared cache, top repositories, cloud bui
 
 > [!NOTE]
 >
-> All free trial credits granted for the Docker Offload Beta expire after 90
+> All free trial usage granted for the Docker Offload Beta expire after 90
 > days from the time they are granted. To continue using Docker Offload after
-> your credits expire, you can enable on-demand usage at [Docker Home
+> your usage expires, you can enable on-demand usage at [Docker Home
 > Billing](https://app.docker.com/billing).
 
 ## Docker Offload billing
@@ -21,7 +21,7 @@ For Docker Offload, you can view and configure billing on the **Docker Offload**
 page in [Docker Home Billing](https://app.docker.com/billing). On this page, you
 can:
 
-- View your included budget
+- View your included usage
 - View rates for cloud resources
 - Enable or disable on-demand usage
 - Add or change payment methods