CI: Remove -Wstrict-overflow

[falbrechtskirchinger]

Starting in GCC 12, ci_test_gcc generates the following error (in most if not all unit tests):

tests/src/unit-constructor1.cpp: In function ‘nlohmann::detail::dtoa_impl::cached_power nlohmann::detail::dtoa_impl::get_cached_power_for_binary_exponent(int)’:
unit-constructor1.cpp:1585:1: error: assuming signed overflow does not occur when changing X +- C1 cmp C2 to X cmp C2 -+ C1 [-Werror=strict-overflow]

Neither rewriting the offending line nor suppressing the warning in get_cached_power_for_binary_exponent(int) is possible.

This PR removes -Wstrict-overflow from the CI warning flags, which is of dubious value anyway.

[coveralls]

Coverage remained the same at 100.0% when pulling 8d28e71 on falbrechtskirchinger:rm-wstrict-overflow into 6058d9a on nlohmann:develop.

[nlohmann]

Looks good to me.