Make allowed version of freemarker explicit

[FSchumacher]

Description

Make the compatible version of freemarker template engine explicit.

Motivation and Context

The freemarker docs state, that it is not allowed to use Config#getVersion()
directly, to specify the version, that we think we are compatible with.

Therefore, let us specify the current version and update it, as we update
the dependency or the templates.

How Has This Been Tested?

Ran ./gradlew build and grepped for getVersion in src/dist-check/jmeter.log and the following message is gone

$ grep -r Configuration.getVersion src/dist-check/jmeter.log
2020-11-03 16:20:29,732 ERROR f.configuration: DefaultObjectWrapper.incompatibleImprovements was set to the object returned by Configuration.getVersion(). That defeats the purpose of incompatibleImprovements, and makes upgrading FreeMarker a potentially breaking change. Also, this probably won't be allowed starting from 2.4.0. Instead, set incompatibleImprovements to the highest concrete version that's known to be compatible with your application.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.