[Snyk] Security upgrade express from 4.16.4 to 4.21.2

[yunnysunny]

Snyk has created this PR to fix 1 vulnerabilities in the npm dependencies of this project.

Snyk changed the following file(s):

• chapter7/package.json

Vulnerabilities that will be fixed with an upgrade:

	Issue
	Regular Expression Denial of Service (ReDoS) SNYK-JS-PATHTOREGEXP-8482416

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🦉 Regular Expression Denial of Service (ReDoS)