Remove FF_K8S_USE_ENTRYPOINT_OVER_COMMAND feature flag

Description

In 11.4 GitLab Runner introduced a feature flag FF_K8S_USE_ENTRYPOINT_OVER_COMMAND in order to fix issues like #2338 (closed) & #3536 (closed)

The value of image:command: from .gitlab-ci.yml, if defined, was overwriting image's entrypoint. While it was defined as the command understood in docker run way. For K8S args should be used. And there was also inconsistency because if only command was used, then it was overwriting the entrypoint, but if also entrypoint was used then it was being assigned into the right place.

With 12.0 we're fixing the implementation. command, if defined in the yaml file, will be always passed as K8S' args. entrypoint, if defined in the yaml file, will be always passed as K8S' command.

Proposal

Remove FF_K8S_USE_ENTRYPOINT_OVER_COMMAND and start using the command over the entry point when it's defined (basically, the behavior when the feature flag is turned off)

Links to related issues and merge requests / references

Main depreciation issue #3473 (closed) MR introducing feature flag !1010 (merged)