remove "informational" status of HTML validation once HTML is expected to validate :-)

[cfm]

Description

This is a reminder to remove the "informational" status of the HTML validation introduced in #6072...once HTML can be expected to validate on develop and we want to hold contributors' pull requests to that standard.

https://github.com/freedomofpress/securedrop/blob/2ed9fce100aa10fc1382eb491ee71f70045fbe44/.circleci/config.yml#L108-L112

[cfm]

We should revisit this question especially in light of html5validator's dependency on a JDK and https://github.com/validator/validator. The latter in turn depends on Log4j, albeit a version prior to those vulnerable to the Log4shell exploits.