docs: add security-plugin-changes report for v3.0.0

[tkykenmt]

Summary

This PR adds documentation for the Security Plugin breaking changes and enhancements in OpenSearch v3.0.0.

Reports Created

• Release report: docs/releases/v3.0.0/features/security/security-plugin-changes.md
• Feature report: docs/features/security/security-plugin.md

Key Changes in v3.0.0

Breaking Changes:

• Blake2b hash implementation fix (produces different but correct hashes)
• OpenSSL provider removal (JDK 21 requirement)
• Whitelist settings removed in favor of allowlist

Enhancements:

• Optimized Privilege Evaluation for better performance
• CIDR range support in ignore_hosts setting
• Password validation strength improvements
• Stop-replication permission added
• Secure password generator action

Resources Used

• PRs: #5089, #5220, #5224, #4380, #5099, #5119, #5160, #5153
• Docs: https://docs.opensearch.org/3.0/breaking-changes/
• Docs: https://docs.opensearch.org/3.0/install-and-configure/configuring-opensearch/security-settings/

Closes #149