Fix OSGi metadata generation to work on JavaSE < 9 (#1603)

* Explicitly specify `Require-Capability: osgi.ee` to require 1.5+

Specifying disables bnd's auto-generated value which is determined from
the minimum versions of the .class files, including `module-info.class`

* Drop JavaSE-1.5 since that boat sailed a long time ago

* Drop unused org.apache.felix:maven-bundle-plugin plugin

gson/bnd.bnd

@@ -3,7 +3,8 @@ Bundle-Name: ${project.name}
 Bundle-Description: ${project.description}
 Bundle-Vendor: Google Gson Project
 Bundle-ContactAddress: ${project.parent.url}
-Bundle-RequiredExecutionEnvironment: J2SE-1.5, JavaSE-1.6, JavaSE-1.7, JavaSE-1.8
+Bundle-RequiredExecutionEnvironment: JavaSE-1.6, JavaSE-1.7, JavaSE-1.8
+Require-Capability: osgi.ee;filter:="(&(osgi.ee=JavaSE)(version=1.6))"
 
 -removeheaders: Private-Package
 

gson/pom.xml

@@ -52,10 +52,6 @@
           </archive>
         </configuration>
       </plugin>
-      <plugin>
-        <groupId>org.apache.felix</groupId>
-        <artifactId>maven-bundle-plugin</artifactId>
-      </plugin>
       <plugin>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>templating-maven-plugin</artifactId>