add announcement for upcoming stdlib migration

news/2024-03-22-stdlib-migration.md

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+# Upcoming migration for stdlib("c")
+
+Almost since the inception of conda-forge, the baseline version of our standard
+library ("stdlib") for C has not changed. This library comes with extra
+complications because it is an essential part of the operating system, and one
+of the few things that conda/mamba/etc. cannot ship safely.
+
+As the ecosystem has moved on and many packages are starting to require newer
+baseline versions, we need to follow suit at some point. However, to avoid
+breaking users on older systems, we need to have infrastructure in place that
+allows our packages to have sufficiently accurate metadata, such that conda can
+avoid the installation of a package requiring a newer stdlib on an old system.
+
+After many discussions across conda-forge stakeholders, the solution we arrived
+at is the introduction of a new Jinja2-function `{{ stdlib("c") }}`, which
+reflects that a given recipe requires a C stdlib. Making this relationship
+explicit will make it easy to correctly reflect the requirement for newer
+stdlib versions per feedstock, as well as in our global pinning.
+
+Up until now, the stdlib was handled implicitly as part of the compiler stack.
+In order to allow this transition to happen, we need to introduce this function
+to essentially all compiled recipes. This will be done in stages, first for a
+single migration, and then attached to _all_ ongoing migrations in conda-forge.
+
+The logic of the piggyback migrator tries to correctly handle most scenarios,
+but it is impossible to cover all corner cases. As for some general rules that
+all feedstock maintainers are free to apply independently:
+
+- if a feedstock uses a `- {{ compiler(...) }}` jinja in the build section,
+  add a line with `- {{ stdlib("c") }}` to the build environment.
+- if a feedstock uses `- sysroot_linux-64 2.17  # [linux64]` (or a variation),
+  remove this line and add the following to your `conda_build_config.yaml`:
+  ```
+  c_stdlib_version:              # [linux]
+    - 2.17                       # [linux]
+  ```
+- if a feedstock sets `MACOSX_DEPLOYMENT_TARGET` in `conda_build_config.yaml`,
+  for example to 10.13 for `x86_64`, replace that section with the following
+  (note, this does _not_ apply to `MACOSX_SDK_VERSION`!):
+  ```
+  c_stdlib_version:              # [osx and x86_64]
+    - 10.13                      # [osx and x86_64]
+  ```
+- In `meta.yaml`, you can then remove any variations of `- __glibc >=2.17` or
+  `- __osx >={{ MACOSX_DEPLOYMENT_TARGET }}  # [osx and x86_64]`, as this will
+  henceforth be handled through `- {{ stdlib("c") }}`.
+
+As these mechanisms begin rolling out, we will also update the maintainer
+documentation in the conda-forge knowledge base. For more details, see this
+[issue](https://github.com/conda-forge/conda-forge.github.io/issues/2102).