chore(deps): update gatsby monorepo to v3.15.0

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
gatsby (source, changelog)	3.14.6 -> 3.15.0
gatsby-plugin-catch-links (source)	3.14.0 -> 3.15.0

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Release Notes

gatsbyjs/gatsby (gatsby)

### [`v3.15.0`](https://togithub.com/gatsbyjs/gatsby/compare/1acb1bcf0f65c3c883c7ff82d39e273744212b65...c1e67a23e298811afa9273a9698af17aec74589d) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/1acb1bcf0f65c3c883c7ff82d39e273744212b65...c1e67a23e298811afa9273a9698af17aec74589d)

gatsbyjs/gatsby (gatsby-plugin-catch-links)

### [`v3.15.0`](https://togithub.com/gatsbyjs/gatsby/compare/f3f1bbc4b75e4ede40500e899449820c958a72ff...c1e67a23e298811afa9273a9698af17aec74589d) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/f3f1bbc4b75e4ede40500e899449820c958a72ff...c1e67a23e298811afa9273a9698af17aec74589d)

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Configuration

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