remove gpg verification of offline-installer package from the documentation

[MinerYang]

We may need remove gpg verification of offline-installer package from the documentation per this discussion
https://goharbor.io/docs/2.12.0/install-config/download-installer/

[DeeBi9]

@MinerYang Can I pick this up

[MinerYang]

@DeeBi9 ,
Sure please help to raise a PR at our https://github.com/goharbor/website and we will review it weekly.

Appreciate!

[DeeBi9]

Hi @MinerYang , I notice in the v2.12.0 from this comment that there is no .asc file for either the offline and online installer and from this comment the gpg verification was skipped due to an expired key.

You mentioned in the issue that GPG verification should be removed for the offline installer. However, since there is no .asc file for either of the installers in this release, I wanted to clarify whether the GPG verification should also be removed for the online installer as well?

Thanks!