rpcclient: Replace strings.SplitN with strings.Cut in cookie file

[healthyyyoung]

Change Description

This PR updates the cookie file parsing logic in rpcclient to use strings.Cut instead of strings.SplitN. The change offers several benefits:

1. Improved code readability with more idiomatic Go
2. Better performance by avoiding temporary slice allocation
3. Adopts the recommended Go 1.18+ approach for string splitting

Steps to Test

Steps for reviewers to follow to test the change.

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.

📝 Please see our Contribution Guidelines for further guidance.