Patch releases targeting the three versions covered by the maintenance
policy and including bug and security fixes will be the default patch
release. However there might be patch releases targeting a specific
version, this logic resides in the `PatchRelease::Issue` class.

This commit performs a cleanup on the `PatchRelease::Issue` class by:
* Dropping the maintenance policy methods. Those were designed to
  account patch releases with only bug fixes
* Adjust the single_version_patch template to only refer to one version

Also this commmit modifies the `Task::Release::Issue` to always create a
patch task for the current version if no version is set.

Related to gitlab-com/gl-infra/delivery#20173