Update the eleven labs to pay less money

We were advised to change the model, we can just merge this before the public release to not overpay.

src/eleven_labs.js

@@ -37,7 +37,7 @@ export class ElevenLabsClient {
   async createSpeech({
     text,
     voiceId,
-    modelId = "eleven_multilingual_v2",
+    modelId = "eleven_flash_v2",
     outputFormat = "mp3_44100_128",
     voiceSettings = null,
     pronunciationDictionaryLocators = null,