Update breaking changes doc for version 25.1

Document the incompatibility of version 25.1 with earlier versions, in
particular, the new minimum Python version requirement (3.11) and the use of
Python 3.13 for development.

docs/breaking_changes.rst

@@ -22,6 +22,18 @@
 Changes That Break Backward Compatibility
 *****************************************
 
+.. _BreakingChangesVersion25.1:
+
+Version 25.1 (unreleased)
+=========================
+
+The following changes that were introduced in version 25.1 of the ``tools`` repository
+are incompatible with earlier versions:
+
+* The minimum supported version of Python is 3.11.
+  Development of the :py:obj:`SalishSeaTools` package is done using Python 3.13.
+
+
 .. _BreakingChangesVersion24.1:
 
 Version 24.1 (2025-01-09)