ci: Remove Linux 5.8 configuration (#24)

### Motivation

Upstream has removed the 5.8 configuration, so setting it causes the runners to fail.

apple/swift-nio@c16420c#diff-c91058a32040621ac54d6ea4194b302a420b7849f6e8561920d23a0e97acf69a

### Modifications

Remove Swift 5.8 configuration, add Swift 6.0 configuration.

### Result

Actions will run again.   Currently they fail with 'startup error'.

### Test Plan

Check that actions-based tests can start again.

Author: euanh

.github/workflows/pull_request.yml

@@ -18,9 +18,9 @@ jobs:
         name: Unit tests
         uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
         with:
-            linux_5_8_enabled: false
             linux_5_9_enabled: false
             linux_5_10_enabled: false
+            linux_6_0_arguments_override: "--skip SmokeTests"
             linux_nightly_6_0_arguments_override: "--skip SmokeTests"
             linux_nightly_main_arguments_override: "--skip SmokeTests"