Unify main.yml and scheduled.yml, remove 5_8 ref (#235)

### Motivation:

* `main.yml` and `scheduled.yml` are mostly duplicative.
* Scheduled runs failed because of a deprecated reference to a Swift 5.8
pipeline

### Modifications:

* Unify `main.yml` and `scheduled.yml`
* Remove the reference to the 5.8 pipeline

### Result:

Working scheduled runs.

.github/workflows/main.yml

@@ -3,13 +3,14 @@ name: Main
 on:
     push:
         branches: [main]
+    schedule:
+        - cron: "0 8,20 * * *"
 
 jobs:
     unit-tests:
         name: Unit tests
         uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
         with:
-            linux_5_8_enabled: false
             linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
             linux_5_10_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
             linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"