fix(deps): update dependency @netlify/serverless-functions-api to v1.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@netlify/serverless-functions-api	1.19.1 -> 1.21.0

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Release Notes

netlify/serverless-functions-api (@​netlify/serverless-functions-api)

### [`v1.21.0`](https://togithub.com/netlify/serverless-functions-api/compare/dd8b017daeac783c552cbd8163dd6ef1be00eef2...6908e6589cbaa13967170de22c5e3595be112ab5) [Compare Source](https://togithub.com/netlify/serverless-functions-api/compare/dd8b017daeac783c552cbd8163dd6ef1be00eef2...6908e6589cbaa13967170de22c5e3595be112ab5) ### [`v1.20.0`](https://togithub.com/netlify/serverless-functions-api/compare/99cf406c4b61d40681ed5981390296f2f64e56a0...dd8b017daeac783c552cbd8163dd6ef1be00eef2) [Compare Source](https://togithub.com/netlify/serverless-functions-api/compare/99cf406c4b61d40681ed5981390296f2f64e56a0...dd8b017daeac783c552cbd8163dd6ef1be00eef2)

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Configuration

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