Isaac Janzen 0e5307d8e2
DEV: Add glimmer header deprecation notice (#25891)
- Remove `_customHeaderClasses`. It was not supposed to have been a part of the glimmer header PR
- Add deprecation notice to `addHeaderPanel`

# Example

<img width="437" alt="Screenshot 2024-02-26 at 2 50 39 PM" src="https://github.com/discourse/discourse/assets/50783505/23fce1ee-fff9-4071-8c34-a480cec7e2e7">
2024-02-28 11:59:44 -07:00
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