Remove "preview" label in Copilot for Xcode readme

**Describe the bug**

It was reported to the GitHub Docs team that "Preview" needs to be removed from the readme here:
https://github.com/github/CopilotForXcode?tab=readme-ov-file#-github-copilot-for-xcode

This is now GA. See: https://github.blog/changelog/2025-02-14-code-completion-in-github-copilot-for-xcode-is-now-generally-available/

**Versions**
- Copilot for Xcode: [e.g. 0.25.0]
- Xcode: [e.g. 16.0]
- macOS: [e.g. 14.6.1]

**Steps to reproduce**
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**Screenshots**

**Logs**

**Additional context**