Add libxml2 2.15.1 to the CI

Author: wetneb

.github/workflows/CI.yml

@@ -31,7 +31,7 @@ jobs:
   test-newer-libxml2:
     strategy:
       matrix:
-        libxml_version: ["2.12.9", "2.13.8","2.14.1"]
+        libxml_version: ["2.12.9", "2.13.8", "2.14.1", "2.15.1"]
     name: With libxml ${{ matrix.libxml_version }}
     runs-on: ubuntu-latest
     steps:
@@ -58,4 +58,4 @@ jobs:
         with:
           command: test
         env:
-          LD_LIBRARY_PATH: /usr/local/lib
+          LD_LIBRARY_PATH: /usr/local/lib