Add Ruby3 to matrix

[CGA1123]

• Add Ruby3 to matrix
• Temporarily disable codecov reporting until it supports Ruby3 (Ruby 3.0 Support & CI Actions improvement codecov/codecov-ruby#124)

Signed-off-by: Christian Gregg [email protected]

[codecov]

Codecov Report

Merging #71 (5656b8f) into master (82dbc56) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #71   +/-   ##
=======================================
  Coverage   82.85%   82.85%           
=======================================
  Files          37       37           
  Lines         700      700           
=======================================
  Hits          580      580           
  Misses        120      120           

Impacted Files	Coverage Δ
lib/slack/block_kit.rb	100.00% <ø> (ø)

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