Add release notes for 2.8.0 (opensearch-project#2827)

Signed-off-by: Ryan Liang <jiallian@amazon.com>

Author: RyanL1997

release-notes/opensearch-security.release-notes-2.8.0.0.md

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+## 2023-06-06 Version 2.8.0.0
+
+Compatible with OpenSearch 2.8.0
+
+### Features
+
+* Identify extension Transport requests and permit handshake and extension registration actions ([#2599](https://github.com/opensearch-project/security/pull/2599))
+* Use ExtensionsManager.lookupExtensionSettingsById when verifying extension unique id ([#2749](https://github.com/opensearch-project/security/pull/2749))
+* Generate auth tokens for service accounts ([#2716](https://github.com/opensearch-project/security/pull/2716))
+* Security User Refactor ([#2594](https://github.com/opensearch-project/security/pull/2594))
+* Add score based password verification ([#2557](https://github.com/opensearch-project/security/pull/2557))
+* Usage of JWKS with JWT (w/o OpenID connect) ([#2808](https://github.com/opensearch-project/security/pull/2808))
+
+### Bug Fixes
+
+* `deserializeSafeFromHeader` uses `context.getHeader(headerName)` instead of `context.getHeaders()` ([#2768](https://github.com/opensearch-project/security/pull/2768))
+* Fix multitency config update ([#2758](https://github.com/opensearch-project/security/pull/2758))
+
+### Enhancements
+
+* Add default roles for SQL plugin: PPL and cross-cluster search ([#2729](https://github.com/opensearch-project/security/pull/2729))
+* Update security-analytics roles to add correlation engine apis ([#2732](https://github.com/opensearch-project/security/pull/2732))
+* Changes in role.yml for long-running operation notification feature in Index-Management repo ([#2789](https://github.com/opensearch-project/security/pull/2789))
+* Rest admin permissions ([#2411](https://github.com/opensearch-project/security/pull/2411))
+* Separate config option to enable restapi: permissions ([#2605](https://github.com/opensearch-project/security/pull/2605))
+
+### Maintenance
+
+* Update to Gradle 8.1.1 ([#2738](https://github.com/opensearch-project/security/pull/2738))
+* Upgrade spring-core from 5.3.26 to 5.3.27 ([#2717](https://github.com/opensearch-project/security/pull/2717))