Modernize

[biow0lf]

Changes:

1. Remove .coveralls.yml
2. Remove .travis.yml
3. Remove Guardfile
4. Remove broken badge from README.md
5. Add GitHub Action actionlint
6. Add GitHub Action rspec
7. Add rubocop config (draft version)
8. Add standard config with target ruby version 3.1
9. Add # frozen_string_literal: true
10. Make ErrbitPlugin:: IssueTracker pure Abstract class. So, #configured?, #errors, #create_issue, #close_issue, #url and .label, . note, . fields and .icons will raise NoMethodError if not redefined in sub-class.
11. Rename ValidateIssueTracker to IssueTrackerValidator.
12. Make ErrbitPlugin::IssueTracker#close_issue required
13. Adjust tests for new code
14. Bump version to 0.7.0

[biow0lf]

Do not merge!

Right now, I will check how its works with new Errbot main.

And do not release!