Add Apple Podcasts icon

[btrem]

This pr adds Apple Podcasts logo. SuperTinyIcons already has iTunes podcasts. However, Apple has renamed iTunes to Apple Music, and renamed iTunes Podcasts to Apple Podcasts. A file name change would normally suffice, but Apple also changed the background color gradient.

This PR adds a new icon with the updated color, and leaves iTunes Podcasts for anyone who wants the old logo.

[btrem]

The failed test result says that the Apple Podcasts reference image is invalid:

1. apple_podcasts.svg should be validated by the w3c validator: AssertionError: expected ':5.54-5.81: error: SVG element “stop”…' to be empty at /home/runner/work/SuperTinyIcons/SuperTinyIcons/test/test.js:34:38

I think whatever validator test.js is running is wrong. In xml, <foo></foo> is equivalent to <foo/>. I chedked the reference image using direct method on the w3c online validator, and it reported that the "document was successfully checked as XML!"