Update pages related to GPG upload from mgradm to mgrctl

[marv7000]

Warning

Do not merge!

Depends on uyuni-project/uyuni#11997 and uyuni-project/uyuni-tools#797

Some hints

Adding an entry to the CHANGELOG.md file in the toplevel directory if necessary.
Cosmetic changes such as fixing typos do not need log entries (nevertheless it is important to fix typos, etc.)!
In the manager-4.3, the hidden .changelog file with a leading dot is still in use.

Add description, target branches, and related links to the section below.

Description

Updated pages mentioning GPG upload via mgradm to the new mgradm API.

Target branches

• Which product version this PR applies to (Uyuni, SUMA 4.3.X, MLM-5.X-MU-A.B.C, or MLM development version). This information can be helpful if ifeval statements are needed to publish it for certain products only.
• Does this PR need to be backported? If yes, create an issue for tracking it and add the link to this PR.
• Whenever possible, cross-reference each backport PR here, so that all backports can be easily accessed from the description.

Backport targets (edit as needed):

• master

Links

• This PR tracks issue https://github.com/SUSE/spacewalk/issues/29690
• Related development PR Add API endpoints for GPG key management uyuni#11997 and Add mgrctl commands for GPG key management uyuni-tools#797