docs(ai): standardize Gordon free-tier terminology (#26420)

## Description

Standardizes the terminology used to describe Gordon’s free tier across
the documentation.

- Replaces **“Personal (free) subscription”** with **“Base tier
(free)”** in the Gordon data privacy documentation.
- Aligns the terminology with the existing Gordon usage-limits
documentation.
- Preserves the existing data retention policy and all related
explanations.

This change improves consistency and clarity for readers without
changing the documented data privacy practices.

## Related issues or tickets

Closes #26419

## Reviews

- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

content/manuals/ai/gordon/concepts/data-privacy.md

@@ -51,7 +51,7 @@ Gordon sessions. Your queries, Gordon's responses, and any code or files
 processed are not stored. This applies to all paid subscriptions: Docker
 Desktop plans (Pro, Team, Business) and Gordon plans (Plus, Max, Ultra).
 
-### Personal (free) subscription
+### Base tier (free)
 
 Anonymized conversation threads are stored for 5 days to help guarantee
 quality of service and fight abuse. Individual queries and responses are