Update upgrades.md with serial=1 for rolling updates (#10837)

* Update upgrades.md

 modify env serial to have real rolling upgrades

* Update upgrades.md

change section for serial

* Update docs/upgrades.md

Co-authored-by: Kundan Kumar <kundan.kumar@india.nec.com>

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Co-authored-by: Kundan Kumar <kundan.kumar@india.nec.com>