Drop Ruby 2.5 support
Just drop Ruby 2.5 support.
With https://gitlab.com/gitlab-org/labkit-ruby/-/jobs/1179430417 and running rake spec
with Ruby 3.0, I found that it does not work at this moment, so leaving it alone to support Ruby 3.0 in this MR.
Blocks !66 (merged)
Closes #15 (closed)
Edited by Takuya Noguchi