Drop bazel 6 in BCR and add --experimental_google_legacy_api for testing

PiperOrigin-RevId: 681081216

.bcr/presubmit.yml

@@ -1,6 +1,6 @@
 matrix:
   platform: ["debian10", "macos", "ubuntu2004", "windows"]
-  bazel: [6.x, 7.x]
+  bazel: [7.x]
 
 tasks:
   verify_targets:
@@ -10,6 +10,7 @@ tasks:
     build_flags:
     - '--host_cxxopt=-std=c++14'
     - '--cxxopt=-std=c++14'
+    - '--experimental_google_legacy_api'
     build_targets:
     - '@protobuf//:protobuf'
     - '@protobuf//:protobuf_lite'
@@ -23,7 +24,7 @@ bcr_test_module:
   module_path: "examples"
   matrix:
     platform: ["debian10", "macos", "ubuntu2004", "windows"]
-    bazel: [6.x, 7.x]
+    bazel: [7.x]
   tasks:
     run_test_module:
       name: "Run test module"
@@ -32,5 +33,6 @@ bcr_test_module:
       build_flags:
       - '--host_cxxopt=-std=c++14'
       - '--cxxopt=-std=c++14'
+      - '--experimental_google_legacy_api'
       build_targets:
       - "//..."