Update greenmail.version to v1.6.15 (#209)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

spring-boot-test-mail-sending/pom.xml

@@ -15,7 +15,7 @@
   <name>spring-boot-test-mail-sending</name>
 
   <properties>
-    <greenmail.version>1.6.5</greenmail.version>
+    <greenmail.version>1.6.15</greenmail.version>
   </properties>
 
   <dependencies>

testing-libraries-overview/pom.xml

@@ -34,7 +34,7 @@
     <microshed-testing.version>0.9</microshed-testing.version>
     <awaitility.version>4.0.3</awaitility.version>
     <rest-assured.version>5.4.0</rest-assured.version>
-    <greenmail.version>1.6.0</greenmail.version>
+    <greenmail.version>1.6.15</greenmail.version>
     <groovy.version>4.0.16</groovy.version>
     <citrus.version>2.8.0</citrus.version>
   </properties>