[Snyk] Security upgrade gatsby from 2.20.12 to 2.20.21

[OJPARKINSON]

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
  • package-lock.json

Vulnerabilities that will be fixed

With an upgrade:

Severity	Priority Score (*)	Issue	Breaking Change	Exploit Maturity
	661/1000 Why? Recently disclosed, Has a fix available, CVSS 7.5	Regular Expression Denial of Service (ReDoS) SNYK-JS-SSRI-1085630	No	No Known Exploit

(*) 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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