Remove unused Go version checks and related test data

[alexandear]

All these files are unused since the tool supports Go 1.24+.

[Copilot]

Pull request overview

Removes legacy Go-version conditional logic and redundant golden test outputs now that the project targets Go 1.24+.

Changes:

• Remove Go version gating/selection logic in cmd/main_e2e_test.go and use t.Name() directly for golden paths.
• Delete Go-version-specific golden files (*-go1.23) for TestE2E_RerunFails.
• Remove unused build-tagged helpers for Go version checks (cmd/goversion_*.go).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cmd/testdata/e2e/expected/TestE2E_RerunFails/reruns_until_success-go1.23	Removes unused Go 1.23-specific expected output.
cmd/testdata/e2e/expected/TestE2E_RerunFails/reruns_continues_to_fail-go1.23	Removes unused Go 1.23-specific expected output.
cmd/testdata/e2e/expected/TestE2E_RerunFails/first_run_has_errors,_abort_rerun-go1.23	Removes unused Go 1.23-specific expected output.
cmd/main_e2e_test.go	Drops Go version branching/skip logic and simplifies golden file selection.
cmd/goversion_old.go	Removes unused pre-go1.22 build-tagged version stub.
cmd/goversion_go122.go	Removes unused go1.22 build-tagged version comparator.

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[dnephin]

Thank you!