Bump default Doxygen version to 1.16.1 in CI workflow

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 237e073..1d2b1a4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@
     branches: [main]
 
 env:
-  DEFAULT_DOXYGEN_VERSION: "1.15.0"
+  DEFAULT_DOXYGEN_VERSION: "1.16.1"
 
 jobs:
   tests: