[Snyk] Security upgrade express from 4.17.1 to 4.21.0

[dmleong]

Snyk has created this PR to fix 4 vulnerabilities in the yarn dependencies of this project.

Snyk changed the following file(s):

• plugins/app-backend/package.json

Note for zero-installs users

If you are using the Yarn feature zero-installs that was introduced in Yarn V2, note that this PR does not update the .yarn/cache/ directory meaning this code cannot be pulled and immediately developed on as one would expect for a zero-install project - you will need to run yarn to update the contents of the ./yarn/cache directory.
If you are not using zero-install you can ignore this as your flow should likely be unchanged.

⚠️ Warning

Failed to update the yarn.lock, please update manually before merging.

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Asymmetric Resource Consumption (Amplification) SNYK-JS-BODYPARSER-7926860	696
	Cross-site Scripting SNYK-JS-EXPRESS-7926867	541
	Cross-site Scripting SNYK-JS-SEND-7926862	391
	Cross-site Scripting SNYK-JS-SERVESTATIC-7926865	391

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Important

• Check the changes in this PR to ensure they won't cause issues with your project.
• Max score is 1000. Note that the real score may have changed since the PR was raised.
• This PR was automatically created by Snyk using the credentials of a real user.

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