Use progress indicator emoji (⏳), not 🦻 to indicate that speech-to-text is happening

🦻 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.

src/controller/chat_completion/mod.rs

@@ -324,7 +324,7 @@ async fn handle_stage_speech_to_text(
         .react_no_fail(
             message_context.room(),
             message_context.event_id().clone(),
-            AgentPurpose::SpeechToText.emoji().to_owned(),
+            strings::PROGRESS_INDICATOR_EMOJI.to_owned(),
         )
         .await;