[Proposal] Rename `IsActuallyPlayingSth` to `IsActuallyPlaying`

[xxSeys1]

Issue description:

The Sth is just weird in a programming context and IsActuallyPlaying tells the user equally as well/ bad what the method is doing.

This breaks the api and also is just a visual thing, so I'd add this in 2.0 or with a new audio system if Flax gets one.

Steps to reproduce:

none

Minimal reproduction project:

none

Flax version:

1.8.1

[mafiesto4]

Good idea to rename it (let's keep old name as deprecated until removal in 2.0).