Bump Markdig from 0.45.0 to 1.4.0

---
updated-dependencies:
- dependency-name: Markdig
  dependency-version: 1.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

src/GitHubIssueManager.csproj

@@ -17,7 +17,7 @@
   <ItemGroup>
     <FrameworkReference Include="Microsoft.AspNetCore.App" />
     <PackageReference Include="Azure.Identity" Version="1.21.0" />
-    <PackageReference Include="Markdig" Version="0.45.0" />
+    <PackageReference Include="Markdig" Version="1.4.0" />
     <!-- Application Insights isn't enabled by default. See https://aka.ms/AAt8mw4. -->
     <!-- <PackageReference Include="Microsoft.ApplicationInsights.WorkerService" Version="2.22.0" /> -->
     <!-- <PackageReference Include="Microsoft.Azure.Functions.Worker.ApplicationInsights" Version="2.0.0" /> -->