Remove multi-PG code

### What

`ProcessGroupManager` has a vector of process groups, `process_groups_`, and iterates through them. This is a remnant from an older design and the current code only supports a single process group. This vector and code handling more than one PG should be removed to simplify the code.
`ProcessGroupStateID` can also be simplified as a `pg_name_` is no longer needed. The config adapter may need updating to support this. In some places, run target name is stored as "/". This identification should also be removed.

### Acceptance Criteria (DoD)

- No more references exist to "process groups" (plural)
- No vectors of or iteration over process groups
- There is no longer a process group identifier or pg name attached to run target names

### How

_No response_