Updating pom.xml to accept Java 8

As suggested by @nbartels: ralfstx#95 (comment)

Author: CLOVIS-AI

.gitignore

@@ -3,4 +3,5 @@ target/
 .history/
 
 # IntelliJ IDEA files
-.idea/
+.idea/
+*.iml

com.eclipsesource.json/pom.xml

@@ -32,13 +32,13 @@
         <configuration>
           <signature>
             <groupId>org.codehaus.mojo.signature</groupId>
-            <artifactId>java15</artifactId>
+            <artifactId>java18</artifactId>
             <version>1.0</version>
           </signature>
         </configuration>
         <executions>
           <execution>
-            <id>ensure-java-1.5-class-library</id>
+            <id>ensure-java-1.8-class-library</id>
             <phase>test</phase>
             <goals>
               <goal>check</goal>
@@ -82,7 +82,7 @@
             <Bundle-SymbolicName>${project.artifactId}</Bundle-SymbolicName>
             <Bundle-Name>${project.name}</Bundle-Name>
             <Bundle-Version>${project.version}</Bundle-Version>
-            <Bundle-RequiredExecutionEnvironment>J2SE-1.5</Bundle-RequiredExecutionEnvironment>
+            <Bundle-RequiredExecutionEnvironment>J2SE-1.8</Bundle-RequiredExecutionEnvironment>
             <Import-Package><!-- without this empty statement, a package import is added --></Import-Package>
             <Export-Package>com.eclipsesource.json;version="${project.version}"</Export-Package>
           </instructions>