C++ client: Fix docs build by locking sphinx to version 7.1.2 for the…

… time being (#4335)

sphinx/sphinx.gradle

@@ -39,9 +39,14 @@ def sphinxDockerfile = tasks.register('sphinxDockerfile', Dockerfile) {
     environmentVariable 'DEEPHAVEN_PROPFILE', 'dh-defaults.prop'
     environmentVariable 'DEEPHAVEN_VERSION', project.version
 
+    // For the time being we lock the sphinx version to 7.1.2
+    // This works around the bug described in
+    // https://github.com/sphinx-doc/sphinx/issues/11605
+    // We should leave this here until 'breathe' updates their code to be
+    // compatible with the breaking sphinx change.
     runCommand '''set -eux; \\
                   mkdir /tmp/workspace; \\
-                  python -m pip install sphinx sphinx-autodoc-typehints pyarrow protobuf grpcio bitstring /wheel/*.whl breathe furo
+                  python -m pip install sphinx==7.1.2 sphinx-autodoc-typehints pyarrow protobuf grpcio bitstring /wheel/*.whl breathe furo
                   '''
 }