[ci] Add Fedora 43 to the CI

Fedora 43 is not released yet, so it's marked as a special build.

The reason why it's added to the CI already early is that it already
provides Python 3.14, which will be released in 3 months. So it's very
useful for testing the ROOT Python interfaces with the upcoming Python
version.

Author: guitargeek

.github/workflows/root-ci-config/buildconfig/fedora43.txt

@@ -0,0 +1,6 @@
+builtin_zlib=ON
+builtin_zstd=ON
+ccache=ON
+test_distrdf_dask=OFF
+test_distrdf_pyspark=OFF
+vdt=OFF

.github/workflows/root-ci.yml

@@ -364,6 +364,9 @@ jobs:
           - image: fedora41
           - image: fedora42
             overrides: ["CMAKE_CXX_STANDARD=23"]
+          - image: fedora43
+            overrides: ["CMAKE_CXX_STANDARD=23"]
+            is_special: true
           - image: alma8
           - image: alma9
             overrides: ["CMAKE_BUILD_TYPE=Debug", "CMAKE_CXX_FLAGS=-Wsuggest-override"]