fix(deps): update dependency koa to v2.15.1

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
koa	2.13.4 -> 2.15.1

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Release Notes

koajs/koa (koa)

### [`v2.15.1`](https://togithub.com/koajs/koa/compare/2.15.0...549455de7898a94b244d42edb0a3c75db478b69a) [Compare Source](https://togithub.com/koajs/koa/compare/2.15.0...2.15.1) ### [`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) ### [`v2.14.2`](https://togithub.com/koajs/koa/compare/2.14.1...2.14.2) [Compare Source](https://togithub.com/koajs/koa/compare/2.14.1...2.14.2) ### [`v2.14.1`](https://togithub.com/koajs/koa/compare/2.14.0...2.14.1) [Compare Source](https://togithub.com/koajs/koa/compare/2.14.0...2.14.1) ### [`v2.14.0`](https://togithub.com/koajs/koa/compare/2.13.4...2.14.0) [Compare Source](https://togithub.com/koajs/koa/compare/2.13.4...2.14.0)

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