Missing default pinning for impi-devel

### Solution to issue cannot be found in the documentation.

- [x] I checked the documentation.

### Issue

Dear conda,

I'm writing because I've been working to implement mpi impi in a number of packages but I'm encountering compatibility issues.
Namely,

In conda-forge, some packages rely on impi v 2021.14, others support the latest 2021.18 etc, despiite the global pinning being 2021.16
https://github.com/conda-forge/conda-forge-pinning-feedstock/blob/d22c5572b0c1e8cd2dc847f16e92743bcd2033e4/recipe/conda_build_config.yaml#L458-L459

Specifically, we added support of impi to
https://github.com/conda-forge/libboost-mpi-feedstock
with
https://github.com/conda-forge/libboost-mpi-feedstock/pull/55

However, the latest version (2021.18.1) is used instead of the global pinning; it would be advisable to fix this behaviour. Do you have ideas on how we should proceed to fix this?

### Installed packages

```shell
libboost-mpi-feedstock
```

### Environment info

```shell
libboost-mpi-feedstock
```

Start with conda_build_config.yaml at lines 458-459 and compare the impi versions used by the linked libboost-mpi-feedstock pull request. Investigate why 2021.18.1 is selected instead of the global 2021.16 pin. Done means impi-devel consistently follows the intended global pinning across affected packages.

Written by the indexing model from the issue text.