Denver Mayor Mike Johnston’s popularity is holding steady after 11 months in office, according to a new poll released Wednesday, but its findings suggest a sales tax increase he’s pitching for the November ballot could face some skepticism from voters. Johnston remains confident in his tax proposal, unveiled Monday. It would generate an estimated $100 million a year to expand on the city’s affordable housing work, including by preserving or building tens of thousands of units affordable to people now getting priced out of the city.