add gcc 11 to CI

.gitlab-ci.yml

CMakeLists.txt

extern/CMakeLists.txt

extern/lodepng/CMakeLists.txt

extern/lodepng/lodepng.cpp

extern/lodepng/lodepng.h

extern/sqlite3/CMakeLists.txt

extern/sqlite3/sqlite3.c

extern/sqlite3/sqlite3.h

src/geometry/CMakeLists.txt

src/geometry/structured/GrayScaleImage.cpp

src/geometry/structured/RGBAImage.cpp

src/pe/CMakeLists.txt

src/pe/communication/rigidbody/Union.h

src/pe/raytracing/Raytracer.cpp

src/sqlite/CMakeLists.txt

src/sqlite/SQLite.cpp

tests/core/cell/CellIntervalTest.cpp