fix(deps): update dependency koa to v2.15.0

[tradeshift-renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
koa	dependencies	minor	2.14.2 -> 2.15.0

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Release Notes

koajs/koa (koa)

### [`v2.15.0`](https://togithub.com/koajs/koa/compare/2.14.2...2.15.0) [Compare Source](https://togithub.com/koajs/koa/compare/2.14.2...2.15.0)

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