chore: fix receiver naming

[Larry-shuo]

replace pp with cpp as the receiver name of ClusterPropagationPolicy:

1. keep the consistency with other place;
2. avoid the confusion with pp of PropagationPolicy.

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1a2c9ff) 33.84% compared to head (b68d09c) 33.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #308 +/- ## ======================================= Coverage 33.84% 33.84% ======================================= Files 149 149 Lines 16682 16682 ======================================= Hits 5646 5646 Misses 10559 10559 Partials 477 477 ``` | [Flag](https://app.codecov.io/gh/kubewharf/kubeadmiral/pull/308/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubewharf) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/kubewharf/kubeadmiral/pull/308/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubewharf) | `33.84% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubewharf#carryforward-flags-in-the-pull-request-comment) to find out more.

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