Exploring Reionization-era Quasars. III. Discovery of 16 Quasars at 6.4 ≲ z ≲ 6.9 with DESI Legacy Imaging Surveys and the UKIRT Hemisphere Survey and Quasar Luminosity Function at z ∼ 6.7

National Key RD National Science Foundation of ChinaNational Natural Science Foundation of China [11533001, 11721303]; US NSF grantNational Science Foundation (NSF) [AST-1515115]; NASA ADAP grant [NNX17AF28G]; National Science Foundation through REU grant [AST-1560461]; NOAO program [GN-2018A-C-1]; ESO Programme [179.A-2010, 179.A-2004]; National Aeronautics and Space AdministrationNational Aeronautics & Space Administration (NASA); National Astronomical Observatories, Chinese Academy of SciencesChinese Academy of Sciences; Special Fund for Astronomy from the Ministry of Finance in China