Removed obsolete dependency to Spring Boot 2.1.5

Author: georgweiss

dependencies/phoebus-target/pom.xml

@@ -8,7 +8,7 @@
   <artifactId>phoebus-target</artifactId>
 
   <properties>
-    <spring.boot-version>2.1.5.RELEASE</spring.boot-version>
+    <spring.boot-version>2.7.3</spring.boot-version>
     <preReleaseScripts>${project.basedir}/release_classpath.py</preReleaseScripts>
   </properties>