[Snyk] Security upgrade gatsby from 2.0.85 to 2.20.21

[snyk-bot]

Snyk has created this PR to fix one or more vulnerable packages in the `npm` dependencies of this project.

Changes included in this PR

• Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
  • package.json

⚠️ Warning

``` Failed to update the package-lock.json, please update manually before merging. ```

Vulnerabilities that will be fixed

With an upgrade:

Severity	Priority Score (*)	Issue	Breaking Change	Exploit Maturity
	758/1000 Why? Proof of Concept exploit, Recently disclosed, Has a fix available, CVSS 7.3	Prototype Pollution SNYK-JS-Y18N-1021887	No	Proof of Concept

(*) Note that the real score may have changed since the PR was raised.

Check the changes in this PR to ensure they won't cause issues with your project.

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