Support Go 1.21 and drop 1.19

[Yang-33]

I found 1.19 was EOL and 1.21 was released : https://endoflife.date/go

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5f21563) 78.89% compared to head (4ac0de4) 78.89%.
Report is 1 commits behind head on master.

❗ Current head 4ac0de4 differs from pull request most recent head 9c67249. Consider uploading reports for the commit 9c67249 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #383   +/-   ##
=======================================
  Coverage   78.89%   78.89%           
=======================================
  Files          41       41           
  Lines        3714     3714           
=======================================
  Hits         2930     2930           
  Misses        592      592           
  Partials      192      192           

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[tokuhirom]

You may need to update https://github.com/line/line-bot-sdk-go/blob/master/go.mod#L3 too.

[tokuhirom]

could you update go.mod in same PR~

[Yang-33]

Thank you! 9c67249 updated it~