ci: modernize GitHub Actions

[altaywtf]

Changed

• Replaces Travis and the old split test/lint workflows with one CI workflow using current Node 24-compatible Actions.
• Tests the supported Go floor on 1.17.x and current Go on 1.26.x.
• Runs race tests and golangci-lint v2.11.3 in the quality job.
• Updates go.mod to go 1.17, matching the existing golang.org/x/oauth2 dependency floor.
• Adds Dependabot maintenance for Go modules and GitHub Actions, and updates the README badge.
• Cleans up test response writes so the modern lint job passes.
• Fixes streamed response timeout handling so callers can read bodies returned from Client.Do(..., nil) before the timeout context is canceled.
• Closes no-body config write responses explicitly, with coverage for SetAll, Set, and Del, so the streaming fix does not leak response bodies.

Risks

• This makes Go 1.17 the declared minimum instead of the stale 1.13 directive.
• This does not add release publishing because go-putio is tag-consumed as a Go module and has no existing artifact release target.
• The streaming timeout fix changes cleanup timing for streamed bodies: callers must close the body, matching the existing contract.

Complexity

• Neutral: workflows are consolidated; runtime changes are limited to Client.Do timeout cleanup and the config methods that intentionally discard response bodies.

Verification

• mise exec go@1.26.2 -- go mod tidy
• mise exec go@1.26.2 -- go test -v ./...
• mise exec go@1.26.2 -- go test -race -count=1 ./...
• mise exec go@1.26.2 golangci-lint@2.11.3 -- golangci-lint run ./...
• actionlint

[Copilot]

Pull request overview

Modernizes the repository’s CI and maintenance automation by consolidating Go test/lint workflows into a single GitHub Actions pipeline, removing Travis, and aligning tests with stricter linting requirements.

Changes:

• Replace Travis + split GitHub Actions workflows with a single .github/workflows/ci.yml workflow (tests + vet + race + golangci-lint).
• Add Dependabot updates for Go modules and GitHub Actions; update README badge to the new CI workflow.
• Refactor test response writing to check write errors consistently via a shared writeResponse helper.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
zips_test.go	Switch fixture writes to writeResponse helper.
transfers_test.go	Switch fixture writes to writeResponse helper.
friends_test.go	Switch fixture writes to writeResponse helper.
files_test.go	Switch fixture writes to writeResponse helper (also contains two updated JSON fixture lines).
events_test.go	Switch fixture writes to writeResponse helper.
account_test.go	Switch fixture writes to writeResponse helper.
client_test.go	Add writeResponse test helper (and import fmt).
README.md	Replace old workflow badges with a single CI badge.
.travis.yml	Remove legacy Travis CI configuration.
.github/workflows/golangci-lint.yml	Remove legacy lint workflow.
.github/workflows/go-test.yml	Remove legacy test workflow.
.github/workflows/ci.yml	Add consolidated CI workflow (tests/vet/race/lint).
.github/dependabot.yml	Add Dependabot configuration for gomod and GitHub Actions.

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