Deprecate `ReadText` in favor of the new Desktop-compatible TTS mechanism

We now have a shiny improved TTS mechanism that is also compatible with the scheme of Anki Desktop, and the old one has bugs #15423. Since we should work in only one direction, the old one should be deprecated.

We can't do it just by removing the setting because a lot of people would get confused and frustrated. So, we should do it by steps.

What I propose is:

- [ ] Add a warning in 2.17 changelog with a link explaining how to migrate
- [ ] Show a snackbar or a toast in EVERY single card that ReadText runs. I want to be annoying here so people migrate fast
- [ ] Show a similar snackbar if someone tries to enable the setting
- [ ] Hide the setting in new installations
- [ ] Remove it completely in 2.18