Update scene-metadata: remove T (13+) age rating, platform is now 18+

## Summary

Decentraland is now an 18+ platform. The Scene Metadata documentation page still references the **T for Teens (13+)** age rating as a valid option, which is no longer accurate.

## Files to update

- `creator/sdk7/projects/scene-metadata.md`
- `creator-esp/sdk7/projects/scene-metadata.md` (Spanish translation)

## Required changes

In the **Age Rating** section:
- Remove the `T for Teens (13+)` bullet point entirely
- Update the prose that says "write either **T** for teens, or **A** for adults" — remove the T option
- Change the JSON example from `"rating": "T"` to `"rating": "A"`
- Update the section description to reflect that 18+ is the minimum

## Context

Requested by Dawid Krajewski via Slack — Decentraland is now 18+ only.

Read the Age Rating sections in creator/sdk7/projects/scene-metadata.md and creator-esp/sdk7/projects/scene-metadata.md. Update both pages to remove the T rating, describe 18+ as the minimum, and use "A" in the JSON example. Done means the English and Spanish pages no longer present T as a valid option and their examples are consistent.

Written by the indexing model from the issue text.