docs(upgrade): replace NEXT placeholder with v2.0.1 (release/2.0)
Cherry-pick of abfdcae8 from main (MR !275 (merged)). Fixes docs/UPGRADE.md migration matrix header v2.0.0 → NEXT → v2.0.0 → v2.0.1 for the 2.0.1 release.
Cherry-pick of abfdcae8 from main (MR !275 (merged)). Fixes docs/UPGRADE.md migration matrix header v2.0.0 → NEXT → v2.0.0 → v2.0.1 for the 2.0.1 release.