fix: upgrade gatsby from 5.12.4 to 5.12.6

Snyk has created this PR to upgrade gatsby from 5.12.4 to 5.12.6.

See this package in npm:
https://www.npmjs.com/package/gatsby

See this project in Snyk:
https://app.snyk.io/org/sammyfilly/project/2353a930-def9-4bb1-816b-7d4f3cbadabd?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot