Remove containerTemplate (now called stepTemplate) 👠

[bobcatfish]

Changes

In #931 we added stepTemplate in addition to containerTemplate and
this chagne went out with 0.5. In this release we can now remove
containerTemplate completely.

Fixes #977

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices

See the contribution guide for more details.

Double check this list of stuff that's easy to miss:

• If you are adding a new binary/image to the cmd dir, please update
  the release Task and TaskRun to build and release this image

Release Notes

🚨Backwards incompatible change 🚨

Removes `containerTemplate` which was renamed to `stepTemplate`. Last release introduced this change in a backwards compatible manner, now we are making the change to remove `containerTemplate`.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bobcatfish

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [bobcatfish]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha1/task_validation.go	78.2%	77.1%	-1.1
pkg/reconciler/v1alpha1/taskrun/resources/pod.go	87.6%	88.5%	0.9
test/builder/task.go	91.4%	91.2%	-0.2

[afrittoli]

Nice cleanup, thank you

[afrittoli]

/lgtm