Mark DeviceLocalAccountManagedSessionEnabled as deprecated

The policy description was changed to say this policy was deprecated
but the deprecated flag wasn't added.
See https://chromium-review.googlesource.com/c/chromium/src/+/2215811

BUG=1086429

Change-Id: Ib9068ec4d0ad12272b71b9bd554933477f702606
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419431Reviewed-by: Aya Elsayed <ayaelattar@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Aya Elsayed <ayaelattar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809275}

components/policy/resources/policy_templates.json

@@ -9919,6 +9919,7 @@
         'dynamic_refresh': True,
         'per_profile': False,
       },
+      'deprecated': True,
       'example_value': True,
       'id': 463,
       'caption': '''Allow managed session on device''',