Make doxygen only a conda dependency. (#1344)

This PR fixes a devcontainers issue seen by @miscco. The `doxygen` dependency can only be installed with conda, so we can't specify it in `requirements.txt`.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #1344

dependencies.yaml

@@ -111,6 +111,8 @@ dependencies:
           # pre-commit requires identify minimum version 1.0, but clang-format requires textproto support and that was
           # added in 2.5.20, so we need to call out the minimum version needed for our plugins
           - identify>=2.5.20
+      - output_types: conda
+        packages:
           - &doxygen doxygen=1.9.1
   cudatoolkit:
     specific: