leveldb: consistently use c++17

[maflcko]

Required for the compiler bump in #12077

std::filesystem is only guaranteed to be in C++17, so select that consistently for this project.

Ref: https://oss-fuzz-gcb-logs.storage.googleapis.com/log-e7599eac-65b6-45fd-bc0d-1deec0235cea.txt

Step #21 - "compile-afl-address-x86_64": [100%] Built target leveldbutil
Step #21 - "compile-afl-address-x86_64": + for fuzzer in fuzz_db
Step #21 - "compile-afl-address-x86_64": + /src/aflplusplus/afl-clang-fast++ -O1 -fno-omit-frame-pointer -gline-tables-only -Wno-error=enum-constexpr-conversion -Wno-error=incompatible-function-pointer-types -Wno-error=int-conversion -Wno-error=deprecated-declarations -Wno-error=implicit-function-declaration -Wno-error=implicit-int -Wno-error=vla-cxx-extension -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=address -fsanitize-address-use-after-scope -stdlib=libc++ -fno-sanitize=vptr -c ../fuzz_db.cc -o fuzz_db.o -DLEVELDB_PLATFORM_POSIX=1 -std=c++11 -Wall -I/src/leveldb/build/include -I/src/leveldb/ -I/src/leveldb/include
Step #21 - "compile-afl-address-x86_64": �[1m../fuzz_db.cc:43:28: �[0m�[0;1;31merror: �[0m�[1mno type named 'remove_all' in namespace 'std::filesystem'�[0m
Step #21 - "compile-afl-address-x86_64":    43 |     std::__fs::filesystem::remove_all(kDbPath);�[0m
Step #21 - "compile-afl-address-x86_64":       | �[0;1;32m    ~~~~~~~~~~~~~~~~~~~~~~~^
Step #21 - "compile-afl-address-x86_64": �[0m1 error generated.
Step #21 - "compile-afl-address-x86_64": ********************************************************************************
Step #21 - "compile-afl-address-x86_64": Failed to build.
Step #21 - "compile-afl-address-x86_64": To reproduce, run:
Step #21 - "compile-afl-address-x86_64": python infra/helper.py build_image leveldb
Step #21 - "compile-afl-address-x86_64": python infra/helper.py build_fuzzers --sanitizer address --engine afl --architecture x86_64 leveldb
Step #21 - "compile-afl-address-x86_64": ********************************************************************************
Finished Step #21 - "compile-afl-address-x86_64"
ERROR
ERROR: build step 21 "gcr.io/cloud-builders/docker" failed: step exited with non-zero status: 1

[github-actions]

maflcko has previously contributed to projects/leveldb. The previous PR was #11866

[jonathanmetzman]

CC @pwnall