chore: update plug_cowboy

[lusergit]

What this PR does / why we need it:

recently mix deps.audit started to fail due to plug_cowboy being affected by a severe vulnerability.

Additional documentation e.g. usage docs, diagrams, reviewer notes, etc.:

---

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fixes #<issue number>

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depends on #<pr number>
depends on https://github.com/...

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gitGraph
    commit id: "Current master"
    branch feat1
    checkout feat1
    commit id: "feat: add something"
    commit id: "feat: add something else (#100)"
    branch feat2
    checkout feat2
    commit id: "refactor: do something"
    commit id: "fix: solve issue"
    commit id: "feat: add a feature (#101)"
    branch feat3
    checkout feat3
    commit id: "feat: feat without pr number"

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[OmarBrbutovic]

/fast-forward