Format Go code with go fmt ./...; check formatting in CI

[alexandear]

1. Format all code with go fmt ./...
2. Remove the unnecessary "Get dependencies" step in CI.
3. Add "Format" step in CI to prevent future regressions.
4. Update checkout and setup-go to v6 in CI. checkout should be before setup-go to enable caching.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.56%. Comparing base (91d2763) to head (8daa634).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #516   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files         191      191           
  Lines       23675    23675           
=======================================
  Hits        23571    23571           
  Misses         63       63           
  Partials       41       41           

Flag	Coverage Δ
unittests	99.56% <100.00%> (ø)

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