DEVX-6617: Update gemspec JWT dependency and Supported versions

[superchilled]

• Updates the gemspec to change JWT dependency from nexmo-jwt to vonage-jwt
• Updates CI GitHub Action to test on supported Ruby version 3.1

[codecov-commenter]

Codecov Report

Merging #251 (61675b2) into dev (099e925) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #251   +/-   ##
=======================================
  Coverage   97.73%   97.73%           
=======================================
  Files          71       71           
  Lines        1457     1457           
=======================================
  Hits         1424     1424           
  Misses         33       33           

Impacted Files	Coverage Δ
lib/vonage/jwt.rb	100.00% <100.00%> (ø)

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[SecondeJK]

I hope that token is all good!

[superchilled]

I hope that token is all good!

Yep. I had to update the hard-coded token because the new JWT library includes typ and alg in the header rather than the payload, so the encoding is different. I've double checked it to make sure it decodes correctly.