⚙️ [Maintenance]: Pin Process-PSModule workflow to v5.4.2 (#25)

This PR pins the Process-PSModule reusable workflow to a specific
release version for improved stability and reproducibility.

## Summary

- Pinned `PSModule/Process-PSModule` workflow to commit SHA
`a15249813c1d7e1a0e4faa2c936fe4978f03845d` (v5.4.2)
- Added version comment for easy identification of the pinned version

## What's Changed

### Workflow Version Pinning

The reusable workflow reference has been updated from a branch reference
to a commit SHA:

| Before | After |
|--------|-------|
| `@test/important-file-changes` |
`@a15249813c1d7e1a0e4faa2c936fe4978f03845d` (v5.4.2) |

### Benefits

- **Reproducibility**: Builds are now deterministic and won't change
unexpectedly
- **Security**: Protects against supply chain attacks via compromised
branches
- **Stability**: Workflow behavior is locked to a known-good release

## Upgrade Impact

| Impact | Description |
|--------|-------------|
| **Breaking Changes** | None |
| **Required Actions** | None |
| **Behavior Change** | Workflow is now pinned to v5.4.2 instead of
following a branch |

Author: MariusStorhaug

.github/workflows/Process-PSModule.yml

@@ -27,7 +27,6 @@ permissions:
 
 jobs:
   Process-PSModule:
-    uses: PSModule/Process-PSModule/.github/workflows/workflow.yml@test/important-file-changes
+    uses: PSModule/Process-PSModule/.github/workflows/workflow.yml@a15249813c1d7e1a0e4faa2c936fe4978f03845d # v5.4.2
     secrets:
       APIKey: ${{ secrets.APIKey }}
-