Deprecate User Defined Variables, and hide it from the menus

### Expected behavior

There should be a way to initialize variables at the beginning of the test.
For instance:
a) Initialize variables based on properties
b) Set thread-group specific variables

In JMeter, `variable` should have `thread` scope, and it is unexpected for the variables from different threads to interfere.
The name `User Defined Variable` suggests that the config element configures variables, however, it configures **test plan** variables which is completely misleading

### Actual behavior

Currently, `User Defined Variables` are replaced globally in the test plan which results in surprising behaviour. For instance, if several `User Defined Variables` located under different thread groups share a variable name, only one of the values win, and the same value is reused for all the threads.

### See also

* https://github.com/apache/jmeter/issues/4360

### Steps to reproduce the problem

[udv_sharing.zip](https://github.com/apache/jmeter/files/11397627/udv_sharing.zip)

### JMeter Version

5.5

### Java Version

17

### OS Version

_No response_