Update dependency gatsby to v2.21.7

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby (changelog)	dependencies	patch	2.21.5 -> 2.21.7

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Release Notes

gatsbyjs/gatsby

### [`v2.21.7`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.6...gatsby@2.21.7) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.6...gatsby@2.21.7) ### [`v2.21.6`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.5...gatsby@2.21.6) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.5...gatsby@2.21.6)

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