chore: xrootd-5.6.9

Updating the xrootd client to 5.6.9 to mitigate https://github.com/xrootd/xrootd/issues/2082. Uses https://github.com/spack/spack/commit/62132919e1465a40e3522e40278e4bbc8ec32978.

spack-environment/packages.yaml

@@ -364,7 +364,7 @@ packages:
     - '@3.20.0'
   xrootd:
     require:
-    - '@5.5.5'
+    - '@5.6.9'
     - cxxstd=20 -davix +python +scitokens-cpp
   zlib-api:
     require: