Add a warning about PAT auth ignoring username

What does this MR do?

We have placed in our docs that set a requirement to use GitLab username when authenticating with personal access token.

For example:

• https://docs.gitlab.com/ee/user/infrastructure/terraform_state.html#using-a-gitlab-managed-terraform-state-backend-as-a-remote-data-source

The username to authenticate with the data source. If you are using a Personal Access Token for authentication, this is your GitLab username.

• https://docs.gitlab.com/ee/user/packages/container_registry/#authenticate-with-the-container-registry

However, at the moment it is not a hard requirement because of this bug report.

This MR adds a note to this bug report to a Personal Access Tokens page.

This seemed like an organic place to add it as many authentication docs link to this page. By having it here we'll be able to easily remove the warning when/if the bugfix is implemented.

Related issues

SSOT bug report Customer ticket - internal only Issue about terraform state files Issue about docker logins with PAT

Author's checklist

• Follow the:
  • Documentation Guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on the:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.

/cc @atanayno