Add note about GHFM HTML spec being outdated

What does this MR do and why?

Related: Tracking Issue: Implement GLFM scripts per the ... (#361241)

Adds note to https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#various-markdown-specifications about outdated HTML version of GitHub Flavored Markdown Specification.

Completes task in #361241

References

See the GLFM Specification Guide development documentation for much more context on all of this work: https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide

MR acceptance checklist

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