[cc] Add deprecation notice for cc backend (#7651)

Issue: #7629 

### Brief Summary
Adding notice to v1.5 release first to collect user feedback.

Author: ailzhang

python/taichi/lang/misc.py

@@ -369,6 +369,11 @@ def init(arch=None,
         raise KeyError(
             "'default_up' is always the unsigned type of 'default_ip'. Please set 'default_ip' instead."
         )
+
+    if arch == cc:
+        warnings.warn(
+            'ti.cc will be deprecated in favor of TiRT and its C API, if you still need it please let us know at https://github.com/taichi-dev/taichi/issues/7629.',
+            DeprecationWarning)
     # Make a deepcopy in case these args reference to items from ti.cfg, which are
     # actually references. If no copy is made and the args are indeed references,
     # ti.reset() could override the args to their default values.

tests/python/test_deprecation.py

@@ -280,3 +280,10 @@ def run():
             r"fill\(\) on ti.Struct is deprecated, and it will be removed in Taichi v1.6.0."
     ):
         run()
+
+
+@test_utils.test(arch=ti.cc)
+def test_cc_deprecation():
+    ti.reset()
+    with pytest.warns(DeprecationWarning, match=r"ti.cc will be deprecated"):
+        ti.init(arch=ti.cc)