Edit changelog and add 1.5.12 release notes.

Author: jxblum

spring-geode/src/main/resources/changelog.txt

@@ -4,8 +4,17 @@ https://github.com/spring-projects/spring-boot-data-geode
 =========================================================
 
 
+Changes in version 1.5.12 (2022-03-31)
+--------------------------------------
+Aligns SBDG with Spring Boot 2.5.11.
+
+* Emergency release in response to: https://spring.io/blog/2022/03/31/spring-framework-rce-early-announcement,
+and CVE: https://tanzu.vmware.com/security/cve-2022-22965.
+* Upgrades to Spring Framework 5.3.18.
+* Upgrades to Spring Boot 2.5.12.
+
 Changes in version 1.5.11 (2022-03-24)
--------------------------------------
+--------------------------------------
 Aligns SBDG with Spring Boot 2.5.11.
 
 * Adds documentation on the Java version.