[Snyk] Security upgrade express from 4.19.2 to 4.21.0

[zntb]

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

Snyk changed the following file(s):

• phonebook-backend/package.json
• phonebook-backend/package-lock.json

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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