BANGKOK, June 24 : Thailand's central bank upgraded its economic growth forecasts on Wednesday, signalling a modest improvement in the outlook after it held interest rates steady for a second straight meeting.    The Bank of Thailand (BOT) said it expected economic growth to come in at 2.3 per cent this year, higher than a projection of 2.0 per cent made earlier this month, after the government announced planned extra borrowing of 400 billion baht ($12 billion). It forecast growth of 1.5 per