Announce the Python 3.9 deprecation (#5095)

As we agreed on the timing, let's officially announce the plan for
deprecating Python 3.9 at least one release before the actual change.

Author: psss

docs/releases/pending/python.fmf

@@ -0,0 +1,11 @@
+description: |
+    This is the **last release where Python 3.9 is supported** on
+    the test :term:`runner`. The minimal requirement since
+    ``tmt-1.79`` to be released in September 2026 is Python 3.12.
+    Please make sure that you are using a supported Python version
+    on the system running ``tmt`` or use one of our `container
+    images`__. Note that this change does not affect in any way
+    :ref:`guest-compatibility`, i.e. the System Under Test where
+    tests are being executed.
+
+    __ https://quay.io/organization/teemtee/

docs/templates/test-runner-guest-compatibility-matrix.inc.rst.j2

@@ -6,6 +6,8 @@
    Please, do not edit this file, is is rendered from runner-guest-compatibility.yaml
    and test-runner-guest-compatibility-matrix.rst.j2, and all your changes would be overwritten.
 
+.. _guest-compatibility:
+
 Guest Compatibility
 -------------------