Bump C++ standard to C++23

[manumerous]

Summary

• Bumps OCS2's own Bazel copts (OCS2_COPTS, OCS2_TEST_COPTS, OCS2_ROS2_COPTS in bazel/ocs2_cxx_flags.bzl) from the previous C++14/C++17 split to -std=c++23 across the board, and updates the accompanying comments in .bazelrc and ocs2_pinocchio/ocs2_pinocchio_interface/BUILD.bazel to match.
• The C++14/C++17 split existed only because gtest and rclcpp/rosidl headers required at least C++17 while the solver core stayed on C++14 to match the original CMake build; now that everything is on C++23, that split is no longer load-bearing (the three copts lists are kept as separate names for future flexibility, but currently resolve to the same standard).
• External dependencies (Eigen, Boost, googletest, ROS2 packages, etc.) are untouched — no repo-wide --cxxopt is set, so they stay on their own toolchain default.

Test plan

• bazel build //... — full build succeeds (9488 actions, only pre-existing -Wsign-compare/-Wunused-variable warnings)
• bazel test //... — 32/32 tests pass
• Verified on GCC 13.3.0 (Ubuntu 24.04)