[release-v0.12.x] Enable Finalizer for Pruner - Grace Period Config

[tekton-robot]

This is an automated cherry-pick of #888

/assign khrm

Enable Finalizer for Pruner - Grace Period Config

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: khrm

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/results/blob/release-v0.12.x/OWNERS)~~ [khrm] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[khrm]

/test pull-tekton-results-unit-tests

[khrm]

/lgtm