Remove `artifact_v4_upgrade` feature flag logic and related PR checks #2927

Job	Run time
Resolve environment (ubuntu-latest, default)	50s
Resolve environment (macos-latest, default)	2m 38s
Resolve environment (windows-latest, default)	3m 12s
Resolve environment (ubuntu-latest, linked)	50s
Resolve environment (macos-latest, linked)	2m 39s
Resolve environment (windows-latest, linked)	3m 30s
Resolve environment (ubuntu-latest, nightly-latest)	48s
Resolve environment (macos-latest, nightly-latest)	2m 57s
Resolve environment (windows-latest, nightly-latest)	3m 24s
	20m 48s

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