Prepare tooling for development

this old man and this seat

[x] create .ruby-version for most recent stable ruby (3.2.2, see https://www.ruby-lang.org/en/downloads/)
[x] use minitest ( + spec + autorun + ???)
[x] rake (?) task for running specs
[x] rubocop
[x] .github/workflow Actions CI config
[x] get CI happy
- [x] use more recent ruby-setup version to pick up ruby-3.2.2 (5cfe23c062c0aac352e765b1b7cc12ea5255ccc4)
- [x] update platforms in Gemfile
- [x] preference: drop ruby 3.1, as I don't really care
[x] script/bootstrap
[x] script/test
[x] rubocop-minitest
[x] update README

rick / branch-deploy