Bump actions/setup-dotnet from v1 to v1.8.0 (restsharp#1579)

Bumps [actions/setup-dotnet](https://github.com/actions/setup-dotnet) from v1 to v1.8.0.
- [Release notes](https://github.com/actions/setup-dotnet/releases)
- [Commits](actions/setup-dotnet@v1...a71d1eb)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/pull-request.yml

@@ -8,7 +8,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v1
-      - uses: actions/setup-dotnet@v1
+      - uses: actions/setup-dotnet@v1.8.0
         with:
           dotnet-version: '3.1.201'