Discontinued support for Julia in v2.13.0

[axel-lauer]

Support for Julia will be discontinued in v2.13.0

All Julia-related dependencies and diagnostics will be retired in the next release of ESMValTool.

Julia support is becoming a burden with the main issue being that it is not easy to package Julia
as it has its own environment management and expects all code to use their management.
Dropping support for Julia will reduce this burden for future maintenance and releases of ESMValTool.

Up to now, only one recipe uses Julia (https://docs.esmvaltool.org/en/v2.12.0/recipes/recipe_rainfarm.html).
This recipe will be retired in v2.13.0 (see https://docs.esmvaltool.org/en/latest/community/broken_recipe_policy.html).

For a list of related tasks, see ESMValGroup/ESMValTool#4132