Upgrade SpringBoot to 2.0.3 (MID-4353)

This is the first version that contains a fix for our JAR class loading
issue (spring-projects/spring-boot#11367).

Also upgraded spring to 5.0.7, listed as a dependency change
for SB 2.0.3.

build-system/pom.xml

@@ -63,9 +63,8 @@
         <selenium.version>1.0.1</selenium.version>
         <slf4j.version>1.7.25</slf4j.version>
         <logback.version>1.2.3</logback.version>
-        <spring.boot.version>2.0.1.RELEASE</spring.boot.version> <!-- can be replaced by a stock version;
-                ideally after https://github.com/spring-projects/spring-boot/issues/11367 is resolved but it should be OK even before that -->
-        <spring.version>5.0.6.RELEASE</spring.version>
+        <spring.boot.version>2.0.3.RELEASE</spring.boot.version>
+        <spring.version>5.0.7.RELEASE</spring.version>
         <spring.security.version>5.0.6.RELEASE</spring.security.version>
         <cas.client.version>3.5.0</cas.client.version>
         <testng.version>6.8.8</testng.version>