Update dependency @tryghost/release-utils to v0.6.15

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@tryghost/release-utils	0.6.14 -> 0.6.15

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Release Notes

TryGhost/Utils

### [`v0.6.15`](https://togithub.com/TryGhost/Utils/compare/@tryghost/release-utils@0.6.14...@tryghost/release-utils@0.6.15) [Compare Source](https://togithub.com/TryGhost/Utils/compare/@tryghost/release-utils@0.6.14...@tryghost/release-utils@0.6.15)

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