Update Ruby versions used in CircleCI
Update Ruby versions used in CircleCI

Ruby 2.5 dropped out of support at the end of March 2021, and Ruby 3.1
was released on Christmas Day.

Signed-off-by: Chris Sinjakli <711c73f64afdce07b7e38039a96d2224209e9a6c@sinjakli.co.uk>

prometheus/client_ruby