By Susan Mathew and Bansari Mayur Kamdar (Reuters) -European stocks slipped from session highs after a report that Russia’s Gazprom has declared force majeure on some gas supplies to Europe stoked worries about an energy crunch in the continent. The pan-European STOXX 600 index, which had risen as much as 1.5% to hit three-week peaks earlier in the session, cut gains to close 0.9% up. Germany’s DAX ended 0.7% higher, above session lows hit following the report. It had climbed as much as 1.5% in morning trade. The letter seen by Reuters said Gazprom, which has a monopoly on Russian gas exports …