switch conmon-rs.spec.rpkg to conmon-rs.spec

### What happened?

rpkg tool is not supported anymore AFAIK. We should switch to conmon-rs.spec. This would also help in the effort to get conmon-rs into the official distros.

### What did you expect to happen?

NA

### How can we reproduce it (as minimally and precisely as possible)?

NA

### Anything else we need to know?

NA

### conmon-rs version

NA

### OS version

NA

### Additional environment details (AWS, VirtualBox, physical, etc.)

NA

Locate conmon-rs.spec.rpkg and inspect how the repository references it, then check the surrounding packaging or build configuration. Done means the repository uses conmon-rs.spec instead and no longer depends on the unsupported .rpkg filename; verify the affected references or packaging checks.

Written by the indexing model from the issue text.