LONDON, April 21 : Microsoft must face a mass lawsuit alleging it overcharged thousands of British businesses to use Windows Server software on cloud computing services provided by Amazon, Google and Alibaba, a London tribunal ruled on Tuesday. Competition lawyer Maria Luisa Stasi is bringing the case on behalf of nearly 60,000 businesses that run Windows Server on rival cloud platforms. Her lawyers have previously said the claim was worth up to 2.1 billion pounds ($2.8 billion). They argued at