Slack verification (#3)

* Adds verification/authentication of event requests from Slack
* Adds NoOpAppMentionHandler extension point
* Use .NET 6 SDK

+semver:major

Author: johnkors

.github/workflows/CI.yml

@@ -2,7 +2,8 @@ name: CI
 
 on:
   push:
-    branches: [master]
+    branches:
+      - "*"
   pull_request:
     branches: [master]
 
@@ -20,6 +21,10 @@ jobs:
         uses: actions/setup-dotnet@v1
         with:
           dotnet-version: 5.0.x
+      - name: Setup .NET 6
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: "6.0.100-rc.2.21458.9"
       - name: Restore dependencies
         run: dotnet restore source
       - name: Build

.github/workflows/PreRelease.yml

@@ -24,6 +24,11 @@ jobs:
         uses: gittools/actions/gitversion/setup@v0.9.7
         with:
           versionSpec: "5.x"
+      - name: Setup .NET 6
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: "6.0.100-rc.2.21458.9"
+          include-prerelease: true
       - name: Determine Version
         id: gitversion
         uses: gittools/actions/gitversion/execute@v0.9.7

.github/workflows/Release.yml

@@ -24,6 +24,10 @@ jobs:
         uses: gittools/actions/gitversion/setup@v0.9.7
         with:
           versionSpec: "5.x"
+      - name: Setup .NET 6
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: "6.0.100-rc.2.21458.9"
       - name: Determine Version
         id: gitversion
         uses: gittools/actions/gitversion/execute@v0.9.7