fix: Add deprecation notice to Streaming Endpoints

[Copilot]

Streaming Endpoints are being phased out in favor of Streaming Methods. Added a deprecation warning to the documentation to inform users.

• Updated section title to "Streaming Endpoints (Deprecated)" in /docs/06-concepts/15-streams.md
• Added :::warning block to the "Streaming Endpoints" section
• Directs users to the recommended Streaming Methods approach
• Updated introduction to focus only on Streaming Methods, removing mention of deprecated Streaming Endpoints

:::warning
Streaming Endpoints are deprecated and will be removed in a future version of Serverpod. Use [Streaming Methods](#streaming-methods) instead for a simpler and more robust streaming experience.
:::

• Fixes Add deprecation notice to Streaming Endpoints. serverpod#4289

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add deprecation notice to Streaming Endpoints.</issue_title>
<issue_description>Add a deprecation notice to Streaming Endpoints in the documentation.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add deprecation notice to Streaming Endpoints. serverpod#4289

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[SandPod]

@copilot - Can you fix the markdown lint failures?

[Copilot]

@copilot - Can you fix the markdown lint failures?

Fixed in 2e47f40. The lint error was caused by the link fragment #streaming-endpoints being invalid after the section title changed to "Streaming Endpoints (Deprecated)". Updated the link to #streaming-endpoints-deprecated.