Add support for defining `keep` and `keep-since` simultaneously in pruner configuration

[avinal]

Feature request

This feature is already present in Tekton cli, referenced by tektoncd/cli#1471 and implemented by tektoncd/cli#1533. It would be logical to update the operator to support a similar functionality in pruner.

Use case

(excerpt taken from tektoncd/cli#1471)

The pruner is used to automatically claim resources used by pipelinruns and/or taskruns by deleting resources exceeding a particular number or age.

There are two criteria used for deleting the resources.

• keep - delete resources exceeding this number
• keep-since - delete resources older than this number in minutes

Let us assume I have 100 pipelines of which 20 are younger than 5 mins and I want to keep a maximum of 10 pipelines.

pruner:
  keep: 10
  keep-since: 5

[pradeepitm12]

/assign @avinal

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[avinal]

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