Migrate non-MIG A100 RE tests to A100-exclusive subplatform

Summary:
Replace the `subplatform = "A100"` + `mig = "false"` combination with `subplatform = "A100-exclusive"` across gpu-remote-execution test targets, and register `A100-exclusive` in the RE subplatform allowlist in `re_test_utils.bzl`.

This is safe after the previous cfgr commit

Differential Revision: D108299291

Author: johnny90

backends/cuda/tests/TARGETS

@@ -24,8 +24,7 @@ python_unittest_remote_gpu(
     ],
     keep_gpu_sections = True,
     remote_execution = re_test_utils.remote_execution(
-        subplatform = "A100",
-        mig = "false",
+        subplatform = "A100-exclusive",
         platform = "gpu-remote-execution",
     ),
 )