Clearly state web terminal is not supported at all on GitLab.com

What does this MR do?

Explicitly states that the web terminal for CI runners will not work for any runner on GitLab.com – I misunderstood that it was only shared runners that were not supported.

Related issues

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/52611 gitlab-runner#3713

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8866

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8866) first, CE-MR afterwards