docs: Convert the deprecation notice to a Deprecated: admonition

`LessThanComparableOrNoneT` announced its deprecation through a
`Warning: Deprecated` admonition, which renders as an ordinary warning and
so reads like any other caveat.

Write it as a `Deprecated:` admonition instead, so it picks up the
deprecation styling added in this branch, and state the version that
deprecated it in the text, as the deprecations guide requires. The
decorator cannot reach a module-level type variable, so a hand-written
admonition is the only channel there is.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/core/math.py

@@ -45,9 +45,9 @@ def __lt__(self, other: Self, /) -> bool:
 )
 """Type variable for a value that is [`LessThanComparable`][..LessThanComparable] or `None`.
 
-Warning: Deprecated
-    This type variable is deprecated and it will be removed in a future version. Use
-    [`LessThanComparableT`][..LessThanComparableT] instead.
+Deprecated:
+    This type variable is deprecated since v1.4.0 and it will be removed in a
+    future version. Use [`LessThanComparableT`][..LessThanComparableT] instead.
 """