ci: doc-build: update Doxygen download URL

Doxygen only serves the latest couple of releases from their website.
However, all releases (including the old ones) can be downloaded from
Sourceforge.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

.github/workflows/doc-build.yml

@@ -49,7 +49,7 @@ jobs:
       run: |
         sudo apt-get update
         sudo apt-get install -y ninja-build graphviz libclang1-9 libclang-cpp9
-        wget --no-verbose https://www.doxygen.nl/files/doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz
+        wget --no-verbose https://downloads.sourceforge.net/project/doxygen/rel-${DOXYGEN_VERSION}/doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz
         tar xf doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz
         echo "${PWD}/doxygen-${DOXYGEN_VERSION}/bin" >> $GITHUB_PATH