Replace atom_version and friends with electron_version

Editor assessment

The subject precisely captures a project-wide rename: atom_version and related identifiers become electron_version. Because the project itself rebranded from atom to electron, the motivation is self-evident and needs no body; the reader immediately understands both what and why. The refactor type fits, no scope is needed for a cross-cutting rename, and no footers appear. Hygiene is clean, with the phrase and friends compactly indicating the scope of the rename without listing every symbol. A mechanical change communicated as completely as its nature requires, with nothing vague or misleading.