Update gatsby monorepo

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby	dependencies	patch	2.18.20 -> 2.18.22
gatsby-image	dependencies	patch	2.2.37 -> 2.2.38
gatsby-plugin-mdx	dependencies	patch	1.0.66 -> 1.0.67
gatsby-plugin-netlify	dependencies	patch	2.1.30 -> 2.1.31
gatsby-plugin-react-helmet	dependencies	patch	3.1.20 -> 3.1.21
gatsby-plugin-sharp	dependencies	patch	2.3.12 -> 2.3.13
gatsby-source-filesystem	dependencies	patch	2.1.45 -> 2.1.46
gatsby-transformer-sharp	dependencies	patch	2.3.11 -> 2.3.12

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Release Notes

gatsbyjs/gatsby

### [`v2.18.22`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.21...gatsby@2.18.22) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.21...gatsby@2.18.22) ### [`v2.18.21`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.20...gatsby@2.18.21) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.20...gatsby@2.18.21)

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