Update dependency koa to v2.15.1

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
koa	2.15.0 -> 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...549455de7898a94b244d42edb0a3c75db478b69a)

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Configuration

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