Abseil LTS 20260107 deprecates MutexLock(Mutex*) and bare Mutex::ReaderLock() / WriterLock() / *Unlock() — used in re2/dfa.cc

Building RE2 2025-08-12 against Abseil lts_20260107 produces -Wdeprecated-declarations warnings at dfa.cc:1018, 1155, 1162-3, 1170, 1172, 1270, 1733, ... (and friends). The MutexLock ones are trivial (drop &); the RWLocker reader/writer lock flow probably needs restructuring to use Reader/WriterMutexLock RAII helpers since the bare methods are deprecated. Would be nice to land a fix before the next RE2 release.