Refresh Go CI matrix

[kvz]

Why:
The SDK CI matrix should keep the declared runtime floor visible while still testing current Go releases. The existing matrix exhaustively tested old Go versions through 1.24, but did not include the current stable Go release.

What:

• Collapse the matrix to a representative spread: Go 1.15, 1.20, 1.24, 1.25, and 1.26.
• Keep the declared go.mod floor covered.
• Update checkout, setup-go, and Codecov actions to current majors so CI can resolve newer Go versions reliably and avoid stale action runtimes.

Validation:

• go test ./... (with TRANSLOADIT_KEY and TRANSLOADIT_SECRET from ~/code/node-sdk/.env)
• go test ./examples/...
• git diff --check

Migration notes:

• actions/checkout@v6, actions/setup-go@v6, and codecov/codecov-action@v6 use newer Node runtimes. This workflow runs on GitHub-hosted ubuntu-latest runners, which satisfy the runner requirements.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.60%. Comparing base (0e48c69) to head (b6b6455).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #44   +/-   ##
=======================================
  Coverage   82.60%   82.60%           
=======================================
  Files           6        6           
  Lines         345      345           
=======================================
  Hits          285      285           
  Misses         32       32           
  Partials       28       28           

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