Remove `artifact_v4_upgrade` feature flag logic and related PR checks #3404

Job	Run time
Config export (ubuntu-latest, linked)	1m 20s
Config export (macos-latest, linked)	2m 11s
Config export (windows-latest, linked)	3m 13s
Config export (ubuntu-latest, nightly-latest)	1m 18s
Config export (macos-latest, nightly-latest)	1m 58s
Config export (windows-latest, nightly-latest)	3m 30s
	13m 30s

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