[Snyk] Security upgrade html-proofer from 3.19.4 to 4.0.0

[tkyi]

Snyk has created this PR to fix 5 vulnerabilities in the rubygems dependencies of this project.

Snyk changed the following file(s):

• Gemfile
• Gemfile.lock

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Expired Pointer Dereference SNYK-RUBY-NOKOGIRI-10674179	726
	Stack-based Buffer Overflow SNYK-RUBY-NOKOGIRI-10674176	721
	Expired Pointer Dereference SNYK-RUBY-NOKOGIRI-10674184	721
	Out-of-bounds Read SNYK-RUBY-NOKOGIRI-10674192	721
	Stack-based Buffer Overflow SNYK-RUBY-NOKOGIRI-10674188	376

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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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