[doc] Use java highlighting in issue templates by default

.github/ISSUE_TEMPLATE/0rule_violation_false-positive.md

@@ -19,7 +19,7 @@ Please provide the rule name and a link to the rule documentation:
 
 **Code Sample demonstrating the issue:**
 
-```
+```java
 
 ```
 

.github/ISSUE_TEMPLATE/1rule_violation_false-negative.md

@@ -19,7 +19,7 @@ Please provide the rule name and a link to the rule documentation:
 
 **Code Sample demonstrating the issue:**
 
-```
+```java
 
 ```
 

.github/ISSUE_TEMPLATE/2new_rule.md

@@ -17,7 +17,7 @@ assignees: ''
 **Code Sample:** This should include code, that should be flagged by the rule. If possible, the "correct" code
 according to this new rule should also be demonstrated.
 
-```
+```java
 
 ```
 

.github/ISSUE_TEMPLATE/4bug_report.md

@@ -25,7 +25,7 @@ A clear and concise description of what the bug is.
 
 **Code Sample demonstrating the issue:**
 
-```
+```java
 
 ```