add-license-headers strips whitespace

### 🔍 Before submitting the issue

- [X] I have searched among the existing issues
- [X] I am using a Python virtual environment

### 🐞 Description of the bug

The hook strips whitespace on the lines we copy over from before the hook was run. We use rstrip, so I think if I strip only ('\n') instead, that could solve the problem.

This is an issue bc it can fail the hook if the header hasn't changed but the trailing whitespace was removed

### 📝 Steps to reproduce

Add trailing whitespace to a line in a file, and run the hook (pre-commit run --file file_name --verbose)

### 💻 Which operating system are you using?

Windows

### 📀 Which ANSYS version are you using?

_No response_

### 🐍 Which Python version are you using?

3.10

### 📦 Installed packages

```shell
N/A
```