Additional CSS: Allow backdrop-filter property

### What

Add backdrop-filter to allowed CSS properties so it won't be stripped on save

### Why

Backdrop-filter allows for some [neat effects](https://css-tricks.com/almanac/properties/b/backdrop-filter/). [This user](https://wordpress.com/forums/topic/the-backdrop-filter-attribute-doesnt-function-properly-in-the-additional-css/) had a sticky header/navigation that was partially transparent and wanted to blur out any text or images as they scroll under. It works and looks nice in Customizer, but it won't save.

### How

Add the property to the allow list in Jetpack's Custom CSS > CSS Tidy > data.inc.php

Open Jetpack's Custom CSS > CSS Tidy > data.inc.php and find the allow list for CSS properties. Add backdrop-filter there, then verify that Additional CSS containing the property is retained after saving.

Written by the indexing model from the issue text.