Bump GO to version 1.21

[umbynos]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?

bump version of GO

• What is the current behavior?

Currently go-paths-helper needs GO version >= 1.21
when running go mod tidy:
go: github.com/arduino/[email protected] requires [email protected], but 1.20 is requested

• What is the new behavior?

Use GO version 1.21

• Does this PR introduce a breaking change?

no

• Other information:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e52bb00) 19.68% compared to head (1741ed1) 19.68%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #872   +/-   ##
=======================================
  Coverage   19.68%   19.68%           
=======================================
  Files          46       46           
  Lines        3672     3672           
=======================================
  Hits          723      723           
  Misses       2855     2855           
  Partials       94       94           

Flag	Coverage Δ
unit	19.68% <ø> (ø)

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