Add 2021.13 to abi_migration_branches

[traversaro]

The 2021.* version is the one effectively pinned by conda-forge (see conda-forge/conda-forge-pinning-feedstock#6635).

Until conda-forge is actually updated to use tbb 2022, it make sense that ABI migrations are applied to 2021.13 as well, to avoid the need to manually open PRs such as #143 and #137 .

I did not bump the build number as this does not actually change the package output in any way.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found some lint.

Here's what I've got...

For recipe/meta.yaml:

• ❌ In conda-forge.yml: $.bot.abi_migration_branches = [2021.13].

  [2021.13] is not valid under any of the given schemas

  Schema

  {
    "anyOf": [
      {
        "items": {
          "type": "string"
        },
        "type": "array"
      },
      {
        "type": "null"
      }
    ],
    "default": [],
    "title": "Abi Migration Branches"
  }

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/16563858852. Examine the logs at this URL for more detail.}

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.