[FEATURE] add Java binary variable to mcserver _default.cfg

## User Story

As a user and system administrator, I want to add a Java binary variable so that I can define which Java should be used for each mcserver server.

## Basic info

* **Distro:** Ubuntu 18.04
* **Game:** Minecraft Server
* **Command:** mcserver.cfg
* **LinuxGSM version:** v21.1.3

## Further Information

Add a default to the mentioned variable and insert the variable into the preexecutable variable.

## Further Reading

I've applied this change in my fork of the repo (develop) for mcserver, pmcserver and wmcserver server _default.cfg's.
The changes are as following:
add this to # Predefined Parameters
`javabin="/usr/bin/java"`
and insert the defined variable in ## Game Server Directories
```
- preexecutable="java -Xmx${javaram}M -jar"
+ preexecutable="${javabin} -Xmx${javaram}M -jar"
```