Mark policy with future flag to avoid including in documentation

This policy is not ready for usage. Hence, marking it with future
flag so that it is not included in the documentation. When the policy
is ready for use, this flag will be removed.

Bug: 1048607
Change-Id: Iefd022161d5f6710da99c8c1404e7b7eb6190f47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2165748
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763548}

components/policy/resources/policy_templates.json

@@ -14529,6 +14529,7 @@
       'name': 'MinimumChromeVersionEnforced',
       'owners': ['snijhara@google.com'],
       'supported_on': ['chrome_os:84-'],
+      'future': True,
       'device_only': True,
       'type': 'dict',
       'schema': {