DOC-3486: Update AI Assistant end-of-sale copy

[kemister85]

Ticket: DOC-3486

Site: Staging

Changes:

• Updated modules/ROOT/partials/misc/admon-ai-pricing.adoc — replaced the end-of-sale IMPORTANT admonition with updated NOTE copy describing AI Assistant as the legacy solution and TinyMCE AI as the current AI offering.
• Affects all 6 AI Assistant pages that include this partial: ai.adoc, ai-openai.adoc, ai-azure.adoc, ai-bedrock.adoc, ai-gemini.adoc, ai-proxy.adoc.

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Pre-checks:

• Branch is correctly prefixed: hotfix/8/DOC-3486
• modules/ROOT/nav.adoc has been updated (if applicable).
• Files have been included where required (if applicable).
• Files removed have been deleted, not just excluded from the build (if applicable).
• Files added for New product features include a release note entry.
• Major or minor version changes have updated the supported-versions.adoc table.
• Build passes without console errors, warnings, or issues.