Use lastIndexOf in Metrics class to futureproof

Because who's going to remember to change that line...

Spigot-Server-Patches/Paper-Metrics.patch

@@ -15,7 +15,7 @@ decisions on behalf of the project.
 
 diff --git a/src/main/java/com/destroystokyo/paper/Metrics.java b/src/main/java/com/destroystokyo/paper/Metrics.java
 new file mode 100644
-index 000000000..91c2aae03
+index 000000000..9389f9a8c
 --- /dev/null
 +++ b/src/main/java/com/destroystokyo/paper/Metrics.java
 @@ -0,0 +0,0 @@
@@ -614,7 +614,7 @@ index 000000000..91c2aae03
 +                    String javaVersion = System.getProperty("java.version");
 +                    Map<String, Integer> entry = new HashMap<>();
 +                    entry.put(javaVersion, 1);
-+                    map.put("Java " + javaVersion.substring(0, 3), entry);
++                    map.put("Java " + javaVersion.substring(0, javaVersion.lastIndexOf('.')), entry);
 +                    return map;
 +                }));
 +            }