chore: modernize CI and document deprecated ioutil usages

[cburgess]

This commit modernizes CI infrastructure and prepares the codebase for future Go version upgrades by documenting all deprecated ioutil usages.

CI Modernization

• Update actions/checkout from v3 to v6
• Update actions/setup-go from v4 to v6
• Add explicit permissions block for security best practices
• Extend Go version matrix to include 1.22.x through 1.25.x
• Remove Go 1.11.x from CI matrix (go.mod specifies go 1.12)
• Maintain backward compatibility with Go 1.12.x through 1.21.x

Build Constraint Cleanup

Merge parse/testdata/subcommand_1.9.go into subcommands.go. The go1.9 build constraint is obsolete since the minimum supported version (1.12) already supports type aliases. This simplifies the test fixtures.

Deprecated ioutil Documentation

Add TODO comments to all ioutil package usages documenting:

• Which function is deprecated
• The modern replacement (available since Go 1.16)
• Reference to the Go 1.16 release notes

The ioutil package was deprecated in Go 1.16, but the code continues to work correctly. These comments prepare for a future migration when the minimum Go version is raised to 1.16+.

Affected functions and their replacements:

• ioutil.Discard -> io.Discard
• ioutil.TempDir -> os.MkdirTemp
• ioutil.TempFile -> os.CreateTemp
• ioutil.ReadFile -> os.ReadFile
• ioutil.WriteFile -> os.WriteFile
• ioutil.ReadDir -> os.ReadDir

Test Fix

Fix TestBootstrap to use go env GOBIN instead of hardcoded $GOPATH/bin, which may not be set in module-aware mode (Go 1.16+ defaults to module mode).

Compatibility

• Minimum Go version: 1.12 (unchanged, now enforced in CI)
• No API changes
• All existing tests continue to pass

[cburgess]

resolves #534