Deprecated: Using ${var} in strings is deprecated

Problem/Motivation

From php 8.2 ${var} is no longer supported inside double quotes. This will error in PHP 9.

Steps to reproduce

Any admin page with thunder_admin as the admin theme will show the following message:
Deprecated: Using ${var} in strings is deprecated, use {$var} instead in /var/core/web/themes/contrib/thunder_admin/thunder_admin.theme on line 352

Proposed resolution

Change to {$var} pattern

Remaining tasks

Create an MR

User interface changes

None

API changes

None

Data model changes

None