[User Profile] Phone number placeholder color is too dark

ux/trivial

## Background:
- Device / Browser (OS version): Chrome on Mac
- Build: Version Tag: 2022-02-24.0

## Description:
Phone number placeholder color is too dark `#595959`

## Steps to reproduce:
1. Log in to the Authgear Portal and select any project
2. Go to the User management page and view the profile page of any user
3. See phone number placeholder

## Expected result:
Should use a translucent or light gray color, like `#a9a9a9` as in Chrome default.

NB `#605D5C` is being used in other textboxes, e.g. profile links and profile picture URL, suggest to have a consistent placeholder color across the site.

## Screenshots:
Placeholder

Color with Input

Start at the Authgear Portal user management profile page and inspect the phone number placeholder styling, comparing it with the profile links and profile picture URL fields. Update the placeholder to the agreed light, consistent color and verify the result in Chrome on macOS.

Written by the indexing model from the issue text.