Add Ruby 3.0 support and drop 2.4/2.5 support

[tnir]

Add Ruby 3.0 support and drop 2.4/2.5 support.

[codecov]

Codecov Report

Merging #257 (4913c0a) into master (37b35a4) will decrease coverage by 0.88%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   97.76%   96.88%   -0.89%     
==========================================
  Files          19       19              
  Lines        2779     2758      -21     
==========================================
- Hits         2717     2672      -45     
- Misses         62       86      +24     

Impacted Files	Coverage Δ
lib/rufo/formatter.rb	96.62% <0.00%> (-1.22%)	⬇️
lib/rufo/command.rb	89.90% <0.00%> (-0.28%)	⬇️
spec/lib/rufo/command_spec.rb	97.52% <0.00%> (-0.10%)	⬇️
spec/lib/rufo/formatter_spec.rb	94.73% <0.00%> (-0.10%)	⬇️
lib/rufo/dot_file.rb	100.00% <0.00%> (ø)
lib/rufo/settings.rb	100.00% <0.00%> (ø)
lib/rufo/file_list.rb	100.00% <0.00%> (ø)
lib/rufo/file_finder.rb	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 37b35a4...4913c0a. Read the comment docs.

[gingermusketeer]

@tnir Thanks a lot for the PR :)