fix(deps): update gatsby monorepo

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby (changelog)	dependencies	patch	2.24.67 -> 2.24.68
gatsby-image	dependencies	patch	2.4.20 -> 2.4.21
gatsby-plugin-feed	dependencies	patch	2.5.13 -> 2.5.14
gatsby-plugin-netlify	dependencies	patch	2.3.16 -> 2.3.17
gatsby-plugin-react-helmet	dependencies	patch	3.3.12 -> 3.3.13
gatsby-plugin-sharp	dependencies	patch	2.6.38 -> 2.6.39
gatsby-source-contentful	dependencies	patch	2.3.49 -> 2.3.50
gatsby-transformer-sharp	dependencies	patch	2.5.16 -> 2.5.17

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Release Notes

gatsbyjs/gatsby

### [`v2.24.68`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.24.67...gatsby@2.24.68) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.24.67...gatsby@2.24.68)

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