Update BCR presubmit to use C++17

This reflects the new minimum as of v30 per https://engdoc.corp.google.com/eng/doc/devguide/proto/news/2024-10-02.md#drop-cpp-14

PiperOrigin-RevId: 694507579

.bcr/presubmit.yml

@@ -8,8 +8,8 @@ tasks:
     platform: ${{ platform }}
     bazel: ${{ bazel }}
     build_flags:
-    - '--host_cxxopt=-std=c++14'
-    - '--cxxopt=-std=c++14'
+    - '--host_cxxopt=-std=c++17'
+    - '--cxxopt=-std=c++17'
     build_targets:
     - '@protobuf//:protobuf'
     - '@protobuf//:protobuf_lite'
@@ -30,7 +30,7 @@ bcr_test_module:
       platform: ${{ platform }}
       bazel: ${{ bazel }}
       build_flags:
-      - '--host_cxxopt=-std=c++14'
-      - '--cxxopt=-std=c++14'
+      - '--host_cxxopt=-std=c++17'
+      - '--cxxopt=-std=c++17'
       build_targets:
       - "//..."