Update to a more recent version of clang-format in related job

Version 3.8 of clang-format is terribly old and difficult to install locally on even moderately recent versions of Ubuntu, requiring using docker for a simple CI-compliant local formatting check.

Some repositories may require some reformatting after this but it's so low effort that I'm not sure this requires a tick-tock cycle of adding an optional updated job before making it mandatory and dropping the old one.