[srp] update config for auto-start mode to be enabled by default (#9738)

Config `OPENTHREAD_CONFIG_SRP_CLIENT_AUTO_START_DEFAULT_MODE` is
changed to use `1` by default (unless explicitly overridden in
project configs).

It also updates related test scripts to utilize auto-start mode or
explicitly disable it for manual SRP client control, reflecting this
default change.

tools/otci/tests/test_otci.py

@@ -406,8 +406,6 @@ class TestOTCI(unittest.TestCase):
         self.assertEqual([], server.srp_server_get_hosts())
         self.assertEqual('running', server.srp_server_get_state())
 
-        self.assertFalse(client.srp_client_get_autostart())
-        client.srp_client_enable_autostart()
         self.assertTrue(client.srp_client_get_autostart())
         client.wait(3)
         self.assertTrue(client.srp_client_get_state())