[8.1.x] doc: add versionadded to Stash and StashKey (#12109)

Co-authored-by: Ran Benita <ran@unusedvar.com>

Author: github-actions[bot]

src/_pytest/stash.py

@@ -19,6 +19,8 @@ class StashKey(Generic[T]):
     A ``StashKey`` is associated with the type ``T`` of the value of the key.
 
     A ``StashKey`` is unique and cannot conflict with another key.
+
+    .. versionadded:: 7.0
     """
 
     __slots__ = ()
@@ -61,6 +63,8 @@ class Stash:
         some_str = stash[some_str_key]
         # The static type of some_bool is bool.
         some_bool = stash[some_bool_key]
+
+    .. versionadded:: 7.0
     """
 
     __slots__ = ("_storage",)