[CI] Disable the API breakage check (#647)

### Motivation

This package doesn't have any library products that would need to
maintain API stability, so we don't need to run the CI.

We only have `_OpenAPIGeneratorCore`, which is underscored and not
expected to be API stable (so it can only be used using an `exact:
"..."` constraint). But we don't want to fail CI when we change that API
(right now it's introducing noise in:
#627)

### Modifications

Disable API breakage check.

### Result

Disabled that CI step.

### Test Plan

N/A

.github/workflows/pull_request.yml

@@ -9,7 +9,7 @@ jobs:
         name: Soundness
         uses: apple/swift-nio/.github/workflows/soundness.yml@main
         with:
-            api_breakage_check_enabled: true
+            api_breakage_check_enabled: false
             broken_symlink_check_enabled: true
             docs_check_enabled: true
             format_check_enabled: true