[prometheus-pushgateway] Fix the image registry and repository default values

[ndebuhr]

Fix the image registry and repository values structuring in prometheus-pushgateway to match intuition and all the other charts in this repo.

What this PR does / why we need it

Which issue this PR fixes

• Fixes an out-of-convention and unintuitive values structuring introduced in [prometheus-pushgateway] allow for global.image.registry and global.imagePullSecrets overrides #5597

Special notes for your reviewer

The registry being bundled into the repository value in the default Helm values is incorrect, and while not strictly a functional bug, it's an unintuitive and out-of-repo-conventions approach. Additionally, the _helper for the registry handling assumes the correct values structuring, so this seems like an initial PR oversight/error rather than a design choice.

Submitting this PR to clean this up. This is not a breaking change if folks are using the default values or the global overrides. It is breaking change if folks are using explicit image (but not global) repository or registry values.

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])