A user has requested to add the ability to delete or purge old versions of a template.

Motivation

Sometimes an old version of the template can be unusable due to many factors such as the docker image it refers doesn't exist anymore or have been renamed etc.

Solution

In such cases, when the old versions are practically unusable it's good to allow deleting them from the CLI and UI
e.g.,

coder templates version prune template_name

Originally requested in: #8307 (comment)