Rename job to "release latest"

What does this MR do?

Rename latest job to release latest, for consistency.

Similar "release" jobs have been renamed when doing !271 (merged), but latest has not been aligned with this change.

NOTE: gemnasium-python's CI config needs to be updated after merging this, because it defines a latest 3.9 job that extends latest.

latest 3.9:
  extends:
    - latest
    - .python-3.9-version
  before_script:
    # gemnasium-python only: append suffix to image tag
    - export TMP_IMAGE=$TMP_PYTHON_IMAGE
    - export IMAGE_TAG=latest-python-$DS_PYTHON_VERSION

What are the relevant issue numbers?

None

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug