BRASILIA, April 29 (Reuters) - Brazil’s government on Thursday slashed its debt forecast for this year to 87.2% of gross domestic product from 96.7%, but said it still expects debt to continue rising in the coming years before peaking in 2026 at 88.5% of GDP. The updated outlook was included in a monthly report from the Treasury that showed a central government primary budget surplus of 2.1 billion reais ($391 million) in March, compared with a 3.1 billion reais deficit forecast in a Reuters poll of economists.