Abseil LTS 20250814.1

What's New:

• absl::Mutex now contains lower-case method names like lock() and shared_lock() to align with standard C++ mutex methods. This allows absl::Mutex to be used with std::scoped_lock and friends. The old names are still present but may be removed in a future release.
• The RAII Mutex-locker types like absl::MutexLock, absl::ReaderMutexLock, and friends now accept references to absl::Mutex. The pointer-accepting constructors are now deprecated, and may be removed in a future release.

Breaking Changes:

• Nullability template types, which were deprecated in the May 2025 release, have been removed.
• absl::string_view(nullptr), which is undefined behavior according to the C++ standard, now triggers an assert failure. Note that unless you changed absl/base/options.h, absl::string_view is an alias for std::string_view, so by default you will be inheriting the behavior of your standard library instead of using the Abseil implementation.
• Abseil's hash tables now require a hash function that has a return type with size >= sizeof(size_t).

Baseline: 987c57f
Cherry-pick: d38452e (Patch 1)