fix: restrict taskDefinition for zeebeServiceTasks only

[smbea]

Current Situation

• TaskDefinition property doesn't have the meta.allowedIn field in the zeebe.json

Problem

• For properties without the meta.allowedIn field, it is assumed to be allowed for all elements
• Doesn't make sense since it's only present for zeebe Service Tasks

Solution

• Add zeebeServiceTasks in the meta.allowedIn field for the taskDefinition property in the zeebe.json

[barmac]

Well done!