🦻 is used for another purpose - to denote that a message is one coming
from speech-to-text, by:
- having the bot react to its own speech-to-text transcription message
with the 🦻 emoji when it's posted in a non-thread
- having the bot prefix its speech-to-text transcription messages with
`> 🦻` when it's posted in a thread
⏳ is already used as a progress indicator for other features, so it
makes sense to use it for indicating that speech-to-text is happening for a given audio message as well.
While 🦻 was an even more descriptive illustration of what's actually happening to the audio message
("it's being heard by the bot"), us using the 🦻 emoji for different things didn't seem good.