[Snyk] Security upgrade kramdown from 2.3.2 to 2.5.1

[jamesoff]

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

Snyk changed the following file(s):

• old_docs/Gemfile

⚠️ Warning

``` Failed to update the Gemfile.lock, please update manually before merging. ```

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Uncontrolled Resource Consumption ('Resource Exhaustion') SNYK-RUBY-REXML-7577227	666
	Improper Restriction of XML External Entity Reference ('XXE') SNYK-RUBY-REXML-7814166	624
	Denial of Service (DoS) SNYK-RUBY-REXML-7577228	559
	Regular Expression Denial of Service (ReDoS) SNYK-RUBY-REXML-8309365	559
	Denial of Service (DoS) SNYK-RUBY-REXML-7462086	479

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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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🦉 Denial of Service (DoS)