Update documentation for Thread::threadId() migration in OpenSearch 2.19.0

[dev-docs-github-app-dev]

This PR updates the documentation to reflect the migration from Thread::getId() to Thread::threadId() in OpenSearch 2.19.0. The update provides guidance and context around this important Java compatibility change.

Key additions to the documentation include:

• Detailed explanation of the migration from Thread::getId() to Thread::threadId()
• List of affected components and systems
• Impact assessment and benefits of the change
• Developer guidance for working with the new thread ID implementation
• References to the original code changes in PR #18237

This documentation update helps developers and users understand:

• Why the change was necessary (Java 19+ compatibility)
• What components were affected by the change
• How to properly use thread IDs in code going forward
• The broader impact on OpenSearch's future compatibility

These changes are part of the OpenSearch 2.19.0 release notes and provide important context for developers working with or maintaining OpenSearch code.

[dev-docs-github-app-dev]

This pull request was created by AI Agent. Please review the changes and provide feedback. Context used:

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