[RAC Select]Allow placeholder to be `ReactNode`

### Provide a general summary of the feature here

Title. There doesn't seem to be any reason to limit it to string.

### 🤔 Expected Behavior?

Make it `ReactNode`

### 😯 Current Behavior

It only allows string

### 💁 Possible Solution

_No response_

### 🔦 Context

I'd like to pass some rich text to placeholder.

### 💻 Examples

_No response_

### 🧢 Your Company/Team

_No response_

### 🕷 Tracking Issue

_No response_

No file or test is named in the issue. Start at the RAC Select entry point and locate the placeholder type, then check the existing Select tests or type checks; done means placeholder accepts ReactNode while existing string usage remains valid.

Written by the indexing model from the issue text.