[Select] Use correct CSS variable for placeholder color

[MarvinKlein1508]

Pull Request

📖 Description

This PR changes the CSS variable which is being used to set the text color for the placeholder in FluentSelect.

Every other input uses --input-placeholder-rest for the text color. FluentSelect has been using --neutral-base-color before which has a slight darker gray tone.

Before:

After:

👩‍💻 Reviewer Notes

I have updated the FluentSelectTests.FluentSelect_Placeholder.verified.razor.html unit test to reflect this change.

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• [] I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component