update macOS SDK baseline

docs/maintainer/knowledge_base.md

@@ -1708,9 +1708,9 @@ You need to rerender the feedstock after this change.
 
 ## Requiring newer macOS SDKs
 
-conda-forge uses macOS SDK 10.9 to build software so that they can be deployed to
-all macOS versions newer than 10.9. Sometimes, some packages require a newer SDK
-to build with. While the default version 10.9 can be overridden using the following
+conda-forge uses macOS SDK 10.13 to build software so that they can be deployed to
+all macOS versions newer than 10.13. Sometimes, some packages require a newer SDK
+to build with. While the default version 10.13 can be overridden using the following
 changes to the recipe, it should be done as a last resort. Please consult with
 core team if this is something you think you need.
 
@@ -1719,13 +1719,13 @@ To use a new SDK, add the following in `recipe/conda_build_config.yaml`
 ```yaml
 # Please consult conda-forge/core before doing this
 MACOSX_SDK_VERSION:        # [osx and x86_64]
-  - "10.12"                # [osx and x86_64]
+  - "10.15"                # [osx and x86_64]
 ```
 
 Note that this should be done if the error you are getting says that a header is not
 found or a macro is not defined. This will make your package compile with a newer SDK
-but with `10.9` as the deployment target.
-WARNING: some packages might use features from `10.12` if you do the above due to
+but with `10.13` as the deployment target.
+WARNING: some packages might use features from `10.15` if you do the above due to
 buggy symbol availability checks. For example packages looking for `clock_gettime`
 will see it as it will be a weak symbol, but the package might not have a codepath
 to handle the weak symbol, in that case, you need to update the `c_stdlib_version`
@@ -1737,9 +1737,9 @@ is available only for macOS x.x, then do the following in `recipe/conda_build_co
 ```yaml
 # Please consult conda-forge/core before doing this
 c_stdlib_version:          # [osx and x86_64]
-  - "10.12"                # [osx and x86_64]
+  - "10.15"                # [osx and x86_64]
 MACOSX_SDK_VERSION:        # [osx and x86_64]
-  - "10.12"                # [osx and x86_64]
+  - "10.15"                # [osx and x86_64]
 ```
 
 In `recipe/meta.yaml`, add the following to ensure that the user's system is compatible.
@@ -1761,7 +1761,7 @@ The libc++ library uses Clang availability annotations to mark certain symbols a
 unavailable when targeting versions of macOS that ship with a system libc++
 that do not contain them. Clang always assumes that the system libc++ is used.
 
-The conda-forge build infrastructure targets macOS 10.9 and some newer C++ features
+The conda-forge build infrastructure targets macOS 10.13 and some newer C++ features
 such as `fs::path` are marked as unavailable on that platform, so the build aborts:
 
 ```bash