Consider using PBKDF2 for key derivation

### Your use case

#### What would you like to do?
Switch from using HKDF for key derivation to PBKDF2

#### Why would you like to do it?
I am working on adding support for Element Call in my Flutter based Matrix client. Currently LiveKit's Flutter SDK derives keys using only PBKDF2, so switching to using this algorithm instead would improve compatibility with Flutter based clients

#### How would you like to achieve it?
I'd be willing to open a PR to make the change, but since it would break compatibility with older versions of Element Call, it might be worth discussing a plan for a smooth transition

### Have you considered any alternatives?

I've also opened a request in LiveKit's Flutter SDK to give an option to use HKDF for keys https://github.com/livekit/client-sdk-flutter/issues/974

### Additional context

Is there any additional context as to why HKDF was chosen for Element Call? If it wouldn't make a significant difference switching to PBKDF2, I believe it would be easier to implement this compatibility fix in Element Call as opposed to upstream LiveKit Flutter SDK, but happy to hear any other thoughts