CI: Add stable clobber (#4780)
Clobbers the `stable` branch with the `audited` tag as described in the
[RELEASE.md](https://github.com/paritytech/polkadot-sdk/blob/master/docs/RELEASE.md#clobbering).
Example of the `staging_stable` branch now after force-pushing the
`staging_audited` tag for a few times. The `staging_` prefix has now
been removed and should be ready for normal use.
The only trigger is currently manual, but can easily be set to three
months.
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Signed-off-by: 
Oliver Tale-Yazdi <[email protected]>