fix: only send length of available_quotes (#13077)

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## **Description**

Fixes longstanding issue where the `SWAP_STARTED` event was sending
incorrect data and causing errors in Mixpanel.

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## **Related issues**

Fixes:

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app/components/UI/Swaps/QuotesView.js

@@ -882,7 +882,7 @@ function SwapsQuotesView({
         slippage,
         custom_slippage: slippage !== AppConstants.SWAPS.DEFAULT_SLIPPAGE,
         best_quote_source: selectedQuote.aggregator,
-        available_quotes: allQuotes,
+        available_quotes: allQuotes.length,
         other_quote_selected: allQuotes[selectedQuoteId] === selectedQuote,
         network_fees_USD: weiToFiat(
           toWei(selectedQuoteValue?.ethFee),