Add version guide and EOL notice to azure-spring-data-cosmos CHANGELOG

Add a version mapping table in the CHANGELOG header block to help
customers identify which version line targets their Spring Boot version.
Includes an End of Life notice for the 3.x line (Spring Boot 2.x OSS
support ended November 2023).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jeet1995

sdk/spring/azure-spring-data-cosmos/CHANGELOG.md

@@ -1,5 +1,15 @@
 ## Release History
 
+> **Version Guide:** This package maintains multiple version lines for different Spring Boot releases.
+> See the [Spring Boot Version Support](README.md#spring-boot-version-support) section in the README for version mapping.
+>
+> | Version Line | Spring Boot | Status |
+> |---|---|---|
+> | **7.x** | 3.4+ | Current |
+> | **6.x** | 3.3+ | Active |
+> | **5.x** | 3.0+ | Active |
+> | **3.x** | 2.4–2.7 | **End of Life** — Spring Boot 2.x OSS support ended November 2023. Please upgrade to 5.x or later. |
+
 ### 7.2.0-beta.1 (Unreleased)
 
 #### Features Added