Fix(operator): Gateway/GatewayClass wording

[lmilan]

Description

Fixes #3598

Preview Links

• https://deploy-preview-3836--kongdeveloper.netlify.app/kubernetes-ingress-controller/gateway-api/#unmanaged-gateways
• https://deploy-preview-3836--kongdeveloper.netlify.app/operator/dataplanes/gateway-api/

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

[netlify]

✅ Deploy Preview for kongdeveloper ready!

Name	Link
🔨 Latest commit	1a3aa76
🔍 Latest deploy log	https://app.netlify.com/projects/kongdeveloper/deploys/695e502d90d87e000880c813
😎 Deploy Preview	https://deploy-preview-3836--kongdeveloper.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.