[df] Extend warning period about Experimental module

This commit extends the warning period regarding the move out of Experimental of distributed RDataFrame interfaces. This is done to mitigate potential confusion of users moving to a more recent version of ROOT from multiple versions in the past. The approach aligns with the discussions held during the ROOT Users Workshop 2025.

Author: vepadulano

bindings/distrdf/python/DistRDF/__init__.py

@@ -213,8 +213,8 @@ class _DeprecatedModule(types.ModuleType):
     def __getattribute__(self, name):
         msg_warng = textwrap.dedent(
             """
-            In ROOT 6.36, the ROOT.RDF.Experimental.Distributed module has become just ROOT.RDF.Distributed. ROOT 6.38
-            will remove the 'Experimental' keyword completely, so it is suggested to move to the stable API in user 
+            In ROOT 6.36, the ROOT.RDF.Experimental.Distributed module has become just ROOT.RDF.Distributed. In the
+            future, the 'Experimental' keyword will be removed, so it is suggested to move to the stable API in user 
             code. You can now change lines such as:
             ```
             connection = ... # your distributed Dask client or SparkContext

roottest/python/distrdf/backends/check_explicit_api.py

@@ -104,8 +104,8 @@ def test_warning_message(self, payload):
 
             msg_warng = textwrap.dedent(
                 """
-                In ROOT 6.36, the ROOT.RDF.Experimental.Distributed module has become just ROOT.RDF.Distributed. ROOT 6.38
-                will remove the 'Experimental' keyword completely, so it is suggested to move to the stable API in user 
+                In ROOT 6.36, the ROOT.RDF.Experimental.Distributed module has become just ROOT.RDF.Distributed. In the
+                future, the 'Experimental' keyword will be removed, so it is suggested to move to the stable API in user 
                 code. You can now change lines such as:
                 ```
                 connection = ... # your distributed Dask client or SparkContext