[CI:DOCS] readme: replace GPG with PGP

[emersion]

GPG is the GNU tooling, PGP is the standard format.

Does this PR introduce a user-facing change?

None

[packit-as-a-service]

We were not able to find or create Copr project packit/containers-podman-23569 specified in the config with the following error:

Packit received HTTP 500 Internal Server Error from Copr Service. Check the Copr status page: https://copr.fedorainfracloud.org/status/stats/, or ask for help in Fedora Build System matrix channel https://matrix.to/#/#buildsys:fedoraproject.org.

---

Unless the HTTP status code above is >= 500, please check your configuration for:

1. typos in owner and project name (groups need to be prefixed with @)
2. whether the project name doesn't contain not allowed characters (only letters, digits, underscores, dashes and dots must be used)
3. whether the project itself exists (Packit creates projects only in its own namespace)
4. whether Packit is allowed to build in your Copr project
5. whether your Copr project/group is not private

[rhatdan]

LGTM
/approve
Thanks @emersion

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: emersion, rhatdan

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[baude]

thank you for your submission!

/lgtm