ci: replace HTML5Validator by proof-html

[aparcar]

It seems more popular and maintained.

[aparcar]

@mwarning somehow it doesn't like ||=, any idea why? It seems to be valid...

[mwarning]

@mwarning somehow it doesn't like ||=, any idea why? It seems to be valid...

We can replace it with something less niftly:

config.versions = config.versions || upstream_config.versions;
config.default_version = config.default_version || upstream_config.default_version;

or just

if (!config.versions) {
  config.versions = upstream_config.versions;
}

if (!config.default_version) {
  config.default_version = upstream_config.default_version;
}

(this might be more readable anyway..)