Correct the AI Gateway mTLS introduction version to 19.2

What does this MR do?

Corrects the version history for the mutual TLS section of the AI Gateway installation page from 19.3 to 19.2.

The implementing commit (f65b7454) is contained in the self-hosted-v19.2.0-ee image tag, and this page's own version pairing rule tells an operator on GitLab 19.2 to run exactly that image, so GitLab 19.2 users have the capability today. The MR that introduced the section (!243827 (merged)) drafted the entry as 19.2 and noted it "should be confirmed against the actual AI Gateway release once known"; the entry was changed to 19.3 during review before the image tag existed. The tag now exists and contains the feature, so the page currently tells 19.2 operators a capability they have is unavailable.

  • !243827 (merged) introduced the section with the version noted as unconfirmed.

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