[Kernel] deprecate -c11 and enable it by default

doc/userman/user-changes.tex

doc/userman/user-sources.tex

man/frama-c.1

man/frama-c.1.md

share/analysis-scripts/estimate_difficulty.py

share/libc/stdnoreturn.h

src/kernel_internals/parsing/clexer.mll

src/kernel_internals/typing/cabs2cil.ml

src/kernel_services/plugin_entry_points/kernel.ml

src/kernel_services/plugin_entry_points/kernel.mli

src/plugins/e-acsl/scripts/e-acsl-gcc.sh

src/plugins/e-acsl/tests/bts/issue-eacsl-40.c

src/plugins/variadic/tests/defined/simple.c

tests/libc/stdnoreturn_h.c

tests/rte/invalid_fptr.i

tests/syntax/anonymous_comp_init.i

tests/syntax/bts0769.i

tests/syntax/built.i

tests/syntax/generic.c

tests/syntax/static_assert.c