By Pavel Polityuk KYIV (Reuters) -Ukraine’s government on Tuesday urged people to conserve energy amid relentless Russian strikes that have halved the country’s power capacity, as the United Nations health body warned of a humanitarian disaster in Ukraine this winter. Authorities said millions of Ukrainians, including in the capital Kyiv, could face power cuts at least until the end of March due to the missile attacks, which Ukraine’s national grid operator Ukrenergo said had wreaked “colossal” damage. Temperatures have been unseasonably mild in Ukraine this autumn, but are starting to dip bel…