feat(nimbus): Adds Win10 only existing users (#15636)

Because

- We want to target Windows 10 (only) existing users for this upcoming
[Win10EOS
experiment](https://docs.google.com/document/d/1H7gtIUB51XSJBWCSlvA6fixqN1c00I9zdPViZ8Kpd1Y/edit?tab=t.0)

This commit

- Combines
[PROFILE28DAYS](https://github.com/mozilla/experimenter/blob/main/experimenter/experimenter/targeting/constants.py#L54)
and
[WIN10_NOT_WIN11](https://github.com/mozilla/experimenter/blob/main/experimenter/experimenter/targeting/constants.py#L1128)

Fixes #15635

Author: mimizillaa

experimenter/experimenter/targeting/constants.py

@@ -2446,6 +2446,17 @@ def __post_init__(self):
     application_choice_names=(Application.DESKTOP.name,),
 )
 
+EXISTING_USER_ONLY_WIN10 = NimbusTargetingConfig(
+    name="Existing user on Windows 10 only",
+    slug="existing_user_win10",
+    description="Users with profiles older than 28 days who are on Windows 10",
+    targeting=f"{PROFILE28DAYS} && {WIN10_NOT_WIN11.targeting}",
+    desktop_telemetry="",
+    sticky_required=True,
+    is_first_run_required=False,
+    application_choice_names=(Application.DESKTOP.name,),
+)
+
 NEW_USER_FIVE_BOOKMARKS = NimbusTargetingConfig(
     name="New user (5 bookmarks)",
     slug="new_user_5_bookmarks",