[MM-63513] Turn Cross team search feature flag into a setting option (#30518)

* feat: Add EnableCrossTeamSearch configuration option to ServiceSettings

* feat: Add EnableCrossTeamSearch configuration option to ServiceSettings

* feat: Enable cross-team search by default

* include old FF in client config

server/tests/test-config.json

@@ -47,6 +47,7 @@
         "EnableEmojiPicker": true,
         "PostEditTimeLimit": -1,
         "TimeBetweenUserTypingUpdatesMilliseconds": 5000,
+        "EnableCrossTeamSearch": true,
         "EnablePostSearch": true,
         "MinimumHashtagLength": 3,
         "EnableUserTypingMessages": true,