v1.5 — fix silent module-cache failure

Fixes a real bug present since setup-go's first release: actions/setup-go's own
empty-cache-dependency-path auto-discovery only checks for a bare go.mod at the
workspace root, never next to go-version-file — so every caller with a module in
a subdirectory (the majority of the fleet) has been silently running with zero
module caching since day one (confirmed via real CI logs: "Restore cache
failed: Dependencies file is not found"). cache-dependency-path now defaults to
go-version-file itself when unset. Also fixes misleading @v1 pinning guidance in
README/HUMANS (v1 is a frozen point-release, not a rolling major alias).