docs/windows/policy: use a separate value to track the configuration state of EnableDNSRegistration

Policy editors, such as gpedit.msc and gpme.msc, rely on both the presence and the value of the
registry value to determine whether a policy is enabled. Unless an enabledValue is specified
explicitly, it defaults to REG_DWORD 1.

Therefore, we cannot rely on the same registry value to track the policy configuration state when
it is already used by a policy option, such as a dropdown. Otherwise, while the policy setting
will be written and function correctly, it will appear as Not Configured in the policy editor
due to the value mismatch (for example, REG_SZ "always" vs REG_DWORD 1).

In this PR, we update the DNSRegistration policy setting to use the DNSRegistrationConfigured
registry value for tracking. This change has no effect on the client side and exists solely to
satisfy ADMX and policy editor requirements.

Updates #14917

Signed-off-by: Nick Khyl <nickk@tailscale.com>

docs/windows/policy/tailscale.admx

@@ -231,7 +231,7 @@
         <string>never</string>
       </disabledValue>
     </policy>
-    <policy name="DNSRegistration" class="Machine" displayName="$(string.DNSRegistration)" explainText="$(string.DNSRegistration_Help)" presentation="$(presentation.DNSRegistration)" key="Software\Policies\Tailscale" valueName="EnableDNSRegistration">
+    <policy name="DNSRegistration" class="Machine" displayName="$(string.DNSRegistration)" explainText="$(string.DNSRegistration_Help)" presentation="$(presentation.DNSRegistration)" key="Software\Policies\Tailscale" valueName="DNSRegistrationConfigured">
       <parentCategory ref="Settings_Category" />
       <supportedOn ref="SINCE_V1_84" />
       <elements>