docs: Add release notes for 3.6.8

[devin-ai-integration]

Adds release notes for Prefect 3.6.8 and regenerates API documentation.

Release highlights:

• New prefect flow-run retry CLI command for retrying failed/completed flow runs
• Bug fixes for Docket worker shutdown, schema preprocessing, webhook notifications, and TASK_SOURCE cache policy
• Documentation updates for MCP server security and AWS IAM role assumption

The release notes were generated using just prepare-release 3.6.8 and cleaned up according to release notes conventions. Integration-only PRs (prefect-aws CDK fix, integration release notes) were excluded per standard practice.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • N/A - release notes PR
• If this pull request adds new functionality, it includes unit tests that cover the changes
  • N/A - documentation only
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

Human Review Checklist

• Verify all PRs merged between 3.6.7 and 3.6.8 are accounted for (check full changelog)
• Verify PR Add semaphore to limit concurrent API calls during Kubernetes observer startup #19957 (Kubernetes observer semaphore) is included in Enhancements section
• Confirm categorization of PRs is appropriate
• Confirm excluded integration-only PRs are correct

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Link to Devin run: https://app.devin.ai/sessions/9ca58d77098d4447886950da7412bbae
Requested by: [email protected] (@desertaxle)

[devin-ai-integration]

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