Update dependency gatsby to v2.21.15

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby (changelog)	dependencies	patch	2.21.12 -> 2.21.15

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Release Notes

gatsbyjs/gatsby

### [`v2.21.15`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.14...gatsby@2.21.15) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.14...gatsby@2.21.15) ### [`v2.21.14`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.13...gatsby@2.21.14) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.13...gatsby@2.21.14) ### [`v2.21.13`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.12...gatsby@2.21.13) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.21.12...gatsby@2.21.13)

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