Revert to official asdf-dotnet-core plugin

What does this MR do?

Revert to the official asdf-dotnet-core plugin. This fixes syntax error in dotnet-install.sh. See gitlab-org/gitlab#357259 (closed)

Installing the CLI requested version 3.1.302. Please wait, installation may take a few minutes.
/opt/asdf/installs/dotnet-core/3.1.302/dotnet-install.sh: line 1: $'\r': command not found
/opt/asdf/installs/dotnet-core/3.1.302/dotnet-install.sh: line 2: $'\r': command not found

Prior to this MR, the pipeline of tests/csharp-nuget-dotnetcore fails: https://gitlab.com/gitlab-org/security-products/tests/csharp-nuget-dotnetcore/-/jobs/2269447280#L337

With this MR, the pipeline succeeds: https://gitlab.com/gitlab-org/security-products/tests/csharp-nuget-dotnetcore/-/pipelines/504872218

What are the relevant issue numbers?

gitlab-org/gitlab#357259 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
  • Ensure that all of the ENV vars supported are propagated to Docker containers upon docker run in job definitions
• Conforms to the code review guidelines
• Security reports checked/validated by reviewer