[Snyk] Security upgrade html-proofer from 3.19.1 to 3.19.1

[snyk-bot]

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

Changes included in this PR

• Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
  • Gemfile
  • Gemfile.lock

Vulnerabilities that will be fixed

With an upgrade:

Severity	Issue	Breaking Change	Exploit Maturity
	XML External Entity (XXE) Injection SNYK-RUBY-NOKOGIRI-1726792	No	No Known Exploit

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

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