fixes #27978 chore(Language Variables) : Register the new Locale Vari…

…able Upgrade Task as a Data Task (#29433)

### Proposed Changes
* Registers the `Task240306MigrateLegacyLanguageVariables` Data Task
which migrates the legacy Language Variables living in the .properties
files under the `/assets/messages/` folder into Contentlets of type
`Language Variable`.
* As specified in [this comment in the
ticket](#27978 (comment)),
the Language Variables will be created for the specific base language if
it exists. If it doesn't, the entry will be created for every Locale
with that specific base language.
* The Language Variable Contentlets will be published after the data
task has run.
* If a key coming from the .properties file already exists as a
Contentlet, it will be ignored and the existing Contentlet will be kept
as is.