switch from main to 0.0.7 for swiftlang workflows (#537)

Motivation

* `@main` references to `swiftlang/github-workflows` are deprecated.

Modifications

* Replace `@main` with `@0.0.7` in all workflow files.

Result

* Workflow files reference a pinned version of
`swiftlang/github-workflows`.

Author: rnro

.github/workflows/pull_request.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   soundness:
     name: Soundness
-    uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
+    uses: swiftlang/github-workflows/.github/workflows/soundness.yml@0.0.7
     with:
       license_header_check_project_name: "SwiftNIO"