Revert to logback 1.3.15 to work with Java 8 (#31)

Author: sbabcoc

pom.xml

@@ -31,7 +31,7 @@
     <maven.compiler.release>8</maven.compiler.release>
     <testng.version>7.5.1</testng.version>
     <junit.version>5.12.1</junit.version>
-    <logback.version>1.5.13</logback.version>
+    <logback.version>1.3.15</logback.version>
     <compiler-plugin.version>3.14.0</compiler-plugin.version>
     <surefire-plugin.version>3.5.3</surefire-plugin.version>
     <source-plugin.version>3.3.1</source-plugin.version>

src/test/java/com/github/sbabcoc/logback/testng/ReporterAppenderTest.java

@@ -10,7 +10,7 @@
 import ch.qos.logback.core.Appender;
 import ch.qos.logback.core.CoreConstants;
 import ch.qos.logback.core.appender.AbstractAppenderTest;
-import ch.qos.logback.core.testUtil.DummyEncoder;
+import ch.qos.logback.core.encoder.DummyEncoder;
 import ch.qos.logback.core.encoder.NopEncoder;
 import ch.qos.logback.core.layout.DummyLayout;