Update feedstock channel selection docs

Documentation for channel sources and targets are out of date. This
setting is now managed in conda_build_config

Related
conda-forge/conda-smithy#897
https://github.com/conda-forge/conda-smithy/blob/9f81a068e8f23f2aae670469b473da786d49d8ef/README.md

Co-authored-by: Daniel Ching <carterbox@users.noreply.github.com>

Author: jaimergp

docs/maintainer/conda_forge_yml.md

@@ -241,22 +241,18 @@ allowed here.
 
 ### channels
 
-This represents the channels to grab packages from during builds and
-which channels/labels to push to on anaconda.org after a package
-has been built.  The `channels` variable is a mapping with
-`sources` and `targets`, as follows:
-
-```yaml
-channels:
-  # sources selects the channels to pull packages from, in order.
-  sources:
-    - conda-forge
-    - defaults
-  # targets is a list of 2-lists, where the first element is the
-  # channel to push to and the second element is the label on that channel
-  targets:
-    - ["conda-forge", "main"]
+:::warning
+This parameter has been deprecated. Instead, specify channels in `recipe/conda_build_config.yaml`
+using `channel_sources` and `channel_targets`. Note that all channels go on a single
+line because each line represents a build variant.
+
+```yaml title="recipe/conda_build_config.yaml"
+channel_sources:
+  - mysourcechannel1,mysourcechannel2,conda-forge,defaults
+channel_targets:
+  - target_channel target_label
 ```
+:::
 
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