Add Thread::getId() to Thread::threadId() migration documentation

[dev-docs-github-app-dev]

This PR adds documentation regarding the migration from Thread::getId() to Thread::threadId() in OpenSearch 2.19.0 release notes. The update includes:

Key changes documented:

• Added information about the replacement of deprecated Thread::getId() with Thread::threadId()
• Clarified that this change is for Java 19 compatibility
• Specified the scope of changes to prevent confusion with other getId() methods
• Highlighted the benefits of this update for code maintenance

The documentation helps developers and users understand:

1. The motivation behind the change (Java 19 compatibility)
2. The scope of the changes (only affecting actual Thread usage)
3. That domain-specific getId() methods are not affected
4. The positive impact on code maintainability

Location: release-notes/opensearch-documentation-release-notes-2.19.0.md

This documentation helps users understand the technical changes while maintaining API clarity.

[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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