Reconsider kickstart partition sizing now that Contest test disks are no longer limited to 20 GB

### Background

In #15025, we bumped the `/usr` partition size in several RHEL 8 `anssi_bp28_*` kickstart files (`intermediary`, `enhanced`, `high`) to fix disk space failures during stabilization testing:

```
logvol /usr --fstype=xfs --name=usr --vgname=VolGroup --size=6667 --fsoptions="nodev"
```
→
```
logvol /usr --fstype=xfs --name=usr --vgname=VolGroup --size=6800 --fsoptions="nodev"
```

During review, @comps noted that the historical practice of keeping our kickstarts within a 20 GB total disk footprint was a limitation of the Contest test infrastructure, not a documented requirement of the project. That limitation was fixed in RHSecurityCompliance/contest#559, and Contest now provisions disks with 100 GB of virtual space (expandable further if needed).

### Problem

Our kickstart files under `products//kickstart/` serve (at least) two different purposes that have gotten conflated:

1. **Test fixtures** for stabilization/CI runs (e.g., via Contest) — previously constrained to fit within 20 GB.
2. **Reference examples** shipped to customers, who may use them as a starting point for their own installs. For this use case, partition sizes double as *minimum recommended sizes* to guide RHEL installs.

Because of (1), partition sizes (like `/usr` above) have been kept artificially tight, leading to repeated one-off bumps (e.g., "add a few more MB") whenever a package set grows and a test starts failing on disk space.

### Proposal

Now that the 20 GB test-infra constraint no longer applies, revisit the sizing strategy for upstream kickstart files:

- Decide whether kickstarts should be restructured with more generous default partition sizes/reserves, reducing the frequency of these disk-space-driven patches.
- Clarify (in docs/comments) which purpose (test fixture vs. customer-facing example) drives the sizing for each kickstart, since the two use cases can pull in different directions.

### References

- #15025 (where this discussion originated)
- RHSecurityCompliance/contest#559 (fix that raised Contest's disk size to 100 GB)