Modernize toolchain: bump go.mod and CI to supported versions

[schultz-is]

The module targeted go 1.14 and CI tested EOL Go 1.14-1.17 with v2-era actions. Update to Go's currently-supported line:

• go.mod: go 1.14 -> go 1.25 (the code uses only long-standing stdlib, so this is purely a supported-baseline bump).
• CI matrix: 1.14-1.17 -> 1.25, 1.26.
• Bump actions to current majors: actions/checkout v2 -> v7, actions/setup-go v2 -> v7, golangci/golangci-lint-action v2 -> v9.
• golangci-lint-action v9 runs golangci-lint v2 and no longer bundles Go, so add an explicit setup-go step to the lint job.
• Set cache: false on setup-go; the module has no dependencies (no go.sum), so dependency caching has nothing to cache.